Disclaimer: I'm mostly making observations based on impressions I've gathered from films about this era (P&P and the like) and books that briefly touch on this era (Bill Bryson). So if my impressions are completely off base, I'd love to hear how!
It seems that 18th century England had a much larger nonworking, affluent class of people who seemed to contribute little in the way of tangible, economic benefit like goods or services. While many of these people contributed in forming a pretty excellent society and certainly contributed artistically, I'm confused as to how this system could reasonably be supported.
tl;dr - Regarded as individuals the people in question are non-working, but taken together as a single economic actor corresponding to a family unit they're actually pretty active in the economy.
There are two things to address here: how depictions of the era are going to skew perceptions, and what the actual economic reality of these people was.
Perceptions: Most of the time these depictions of the life of the English upper-class won't really explore the economic basis of these peoples' lives because that's not what they're about. It's been a while since I last watched or read Pride and Prejudice, but isn't the open discussion of business actually called out as being gauche in the settings in which a lot of that story takes place? We don't talk about that in polite company. Even then, somehow they talk about money all the time, who controls the money, how much there is, how people came by their money, all of that. This hints at the reality of the situation.
Realities: As usual, the actual explanation is something that reasonable people could argue about, but the simplest version is that if you consider these non-working individuals as a family unit, they're actually pretty powerful economic actors. The estates of the landed classes didn't change that much between the advent of "feudalism" (yeah, I know, not really a thing) and industrialization, at least not economically; the main changes to look for are the reduction of their explicit role in defense collectivization as the state becomes the custodian of legitimate force, and the diversification of economic interest away from rent-collecting into new areas as they become available.
Another part of the answer is that England has sophisticated financial institutions and reasonably strong rule of law. This is important because it means that the size of economic actors and the scope of their action can increase dramatically because the stakeholders can rely on government institutions to protect their interests. This is especially important to pre-industrial economic activities like trade and commodities speculation that would produce the kind of concentrations of wealth that are regarded by some as a necessary pre-condition to industrial growth. Enclosure would also have dramatically changed the income of the non-working classes, as the value of the land they owned would have increased dramatically once the rights of the open field system no longer had to be respected. THis is actually regarded as another key factor in later industrialization, both because of the concentration of fungible wealth for investment, but also because of the creation of a large population of landless poor.
So what do we get when we put all of that together? What do these economic actors look like? Well, like large families of dilettantes who spend most of their time loafing, playing cards, and making one-another miserable, at least in terms of their daily lives. Most of the money to support this, by the time of the 18th century, will be carefully managed by financial institutions, sometimes with the intervention of one or more members of the estate, sometimes not. The estate, regarded as a unified economic actor, will have its hands in everything that whoever's running the show deems profitable; the money for the daily lives of people like the Bennets comes from the profits of those activities and interest on existing assets. As I suggested above with examples of these interests above, the actions of the estates of the landed classes actually ended up being pretty important in terms of industrialization. While a lot of these people don't work, saying that they don't contribute much economically isn't accurate.
It should also be noted that /u/Second_Mate is quite right that they're actually a pretty small fraction of the population, too.
Sources Moore, Barrington - Social Origins of Dictatorship and Democracy: Lord and Peasant in the Making of the Modern World. Beacon Press, 1966
McCloskey, Donald - The Economics of Enclosure in European Peasants and Their Markets: Essays in Agrarian Economic History (ed. Jones & Parker). Princeton University Press, 1975.
Wallis, Patrick and Webb, Cliff - The Education and Training of Gentry Sons in Early-Modern England. Economic History Working Papers, 128/09. Department of Economic History, London School of Economics and Political Science, 2009.