Why would they? Economics seems emblematic of all that grey mundanity we read and write fantasy to get away from. I'm no different: I have to make an effort not to switch off when the economic news comes on.
Economics, though, isn't an obscure, arcane lore known only to five and a half people (none of whom are in the government). The word actually means housekeeping, and it's part of all our lives. Every time you go into a shop or click on a buy link; every time your employer pays you for the work you've done in the past month; every time you sit down with your bank statement and work out if you'll have enough left this month for that holiday, or to buy the latest series of A Game of Thrones: that's all economics.
More than that, though: economics has been a fundamental part of human behaviour throughout the history of our species. Paleolithic man traded and even had factories *, and finds from at least one of the early Neolithic towns in the middle east have included clay chips that may have been primitive money. Come to that, towns themselves are an economic institution. They might also have a defensive function, but their main purpose is to provide a convenient nexus for crafts and trade.
All the great civilisations from which fantasy draws inspiration, from Egypt under the Pharoahs to Renaissance Europe, were built on an economy. It's no exaggeration to say that you can't have any organised society without economics.
I'm not an economist, and an expert would probably find holes in what follows big enough for a dragon to fly through. I'm not trying to give a lecture, though, merely suggest areas to think about and research further when creating your world.
Broadly speaking, economics divides into three areas. A society must be able to obtain resources, most importantly food, but also minerals, building materials and other things. It must have a system that allows it to acquire and exploit the resources in a mutually beneficial way **. And it must have a system that allows it to exchange its surplus resources for items it can't get directly — in other words, trade.
If you examine a map of the real world, you'll find that there's usually a reason for important towns and cities to be where they are. Often this will be the interface between a fertile hinterland and easy transport for trade. The prosperous civilisation that flourished in the Mississippi basin at the same time as mediaeval Europe, for instance, had its great city at the point where the Mississippi and Missouri meet. Centuries later, the European settlers built St Louis in exactly the same place. The position is ideal, both to dominate an excellent agricultural region and to access all the main waterways in the centre of the continent.
It's not always food, although a city without easy access to food would need something spectacular going for it ***. Mining is always a vital industry, and some other industries thrive in specific locations. The English cotton industry in the Industrial Revolution, for instance, was centred in east Lancashire because the climate and landscape made the area ideal for cotton mills.
Economic richness can have a downside, though. The main reason for the Roman conquest of Britain wasn't self-aggrandising empire-building, it was because Britain is (or was, at least) unusually rich in mineral resources: tin and coal especially (less glamorous than gold, but far more useful) but many others too.
In fantasy novels, empires tend to expand because the king is obsessed with matching the conquests of his forebears, or because their religion has a crusading mission, or because they believe in civilising the barbarians. All of these are certainly reasons for empire-building — for public consumption, certainly — but the underlying reason is almost always because the conquered land has something the empire needs, or at least wants.
Wars are much the same: competition either for resources or to control important trade routes. Again, these aren't the reasons given, the cause used to rouse soldiers to leave their homes and fight to the death. They believe they're fighting for civilisation, for right, for the glory of their nation or their religion. In fact, they're almost always fighting for the economy.
The economy's social contract can be difficult to define, but is perhaps the most ubiquitous aspect. The contract may be a utopian system where each gives what they can and takes what they need, but the chances are it will be very unequal. The European feudal system is often seen as oppression pure and simple — and it was sometimes, especially at the stage when it had outlived its use — but the essential point was that the peasants got to keep a little of their harvest, at least, without the nearest robber-band taking it all from them, and the knights and lords were fed and could specialise in protection. It didn't always work properly, but that was the theory, anyway.
The basic arrangement is about who does the work and who gets what out of it. Even the most wretched peasants or industrial workers get something, if not much, for the simple reason that without enough to eat they can't work. Usually, though, it's more two sided than that.
Your city or kingdom in a fantasy world will have many people living their lives, and sometimes that will impinge on even the most single-minded band of adventurers. The inn they stay at will be owned by someone who has an arrangement (normally financial) with the people who serve the drinks, prepare the food and wash the dishes afterwards. The blacksmith who reshoes the adventurers' horses will have set his prices so he can cover the cost of his materials and have enough left over to buy food for himself and his family; although in a fairly primitive society, he may simply trade his work for the food itself.
Trade is the lifeblood of any civilisation, and the people who pursue it will be important in your fantasy culture, whether they adventure into the wild to obtain something in high demand (like the trappers of North America) or run a shop or market stall to sell goods from afar to their fellow-citizens. Along with local resources, prevailing trades will affect the look and feel of your culture. All those clothes, jewellery, ornaments, weapons and rare spices must have come from somewhere, and if they aren't local, it must be feasible that they could have been brought there by merchants.
This, as I say, isn't a lesson on how it works, just an encouragement to think about an often-neglected aspect of world-making. I'm obviously not suggesting you should draw up a detailed, complex economic plan for your world, any more than I'd suggest fully creating all the languages spoken in it — unless, of course, you really, really love doing that. But, along with the geography and history and religion and customs of your lands and peoples, it's worth giving some thought about how their economies fit together.
Because, in fact, it's intimately related to all the other aspects.
** Of course, the benefits won't necessarily be equal, but even ground-down peasants will normally get something out of the arrangement. The exception is slave-based societies, where a section of the population gets no benefit at all.
*** The royal seat of the Hittite Empire bucks the trend. Build purely for strength, defensibility and impressiveness, it stood high in the mountains, far from any agricultural land or trade routes. However, this was after the tribute had started pouring in from the four fertile corners of the empire, enabling the capital to survive without producing.