Is 'working out' wasting a resource?Let's see how much that electricity is worth.
If you pedal on an exercise bike for half an hour, the bike readout will probably tell you that you burned about 300 food calories (0.3 kWh). Of that, perhaps 75% heats you up while a quarter goes into the bike, so you're putting in about 0.1 kilowatt-hours of electricity. (We will ignore inefficiency in the generator, which is small.) In the US, that's about a cent worth. If the electric company paid you for your effort, you'd be making 0.3% of minimum wage. After two years of daily effort, you'd have made enough to buy one cheap meal, regaining just a fraction of one percent of the energy you put in to begin with.
An average American's energy footprint is about 200 kWh per day (a hundred times a human metabolism), so if a quarter of the population adopted this daily bicycle habit, it would provide just over 0.01% of the nation's energy usage. (And, since this would mean considerably more people riding exercise bikes than currently do, the extra energy used for food, gyms, driving to gyms, etc. would far outweigh the savings and lead to a net increase of energy usage.)
It takes about 60 kWh of electricity to make a kilogram of aluminum. If each generator uses 3kg, it would take 1,000 hours of riding the bike just to make that back. In short, the power generated by humans is nominal; capturing it would have no measurable effect on the economy or environment.
However, capturing energy could indeed make economic sense in theory for a gym owner. If generator units cost $50 each and were used for five hours a day, they'd easily clear a 50% annual gross return. With a lifetime of a decade, they could be said to make economic sense. (There is another effect because the bicycles now give off slightly less heat than they used to, lowering air-conditioning costs and raising heating costs, but let's assume that's a small net effect.)
The absolute size of the return is very small, though, so it could be easily wiped out by other effects. In practice, gyms that have "eco-friendly" equipment have it because of the advertising appeal, not the savings on electricity. People like the idea that they're helping the environment by working out, and are generally unaware of how miniscule the impact is. After "helping the environment" during their workout, they're likely to be less conscious of over-using energy the rest of the day, an effect called Self-licensing.
How much self-licensing would it take to offset the gains? If our hypothetical biker takes a hot shower after riding, all the extra electricity they generated will be used up if they stay in the shower an extra ten seconds.
In short, there's no significant incentive to recapture this energy aside from advertising / signalling, and the orders of magnitude are such that changing energy prices aren't going to change that.