Corretec makes belt driven bikes. They are popular at seaside resorts, and the folks I've spoken to who have them really like them.

That's what I think. We've had 100 years of development time with chain drives so, whatever replaces chain drives, is going to have to be pretty well developed. It needs to be close to the mechanical efficiency of a chain drive system and it needs to be marketable at close to the price of a chain drive system. I don't think they're quite there yet.

Yeah, I doubt they can stand up to the rigors of bicycle use.

Belts have two advantages over chains (lower noise/vibration and less maintenance) and one disadvantage (lower drive efficiency, especially under higher torque loads).

On bicycles, the noise/vibration of chain drives is not an issue--as it is for motorcycles.
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That leaves you balancing the advantage of lower maintenance over the disadvantage of lower drive efficiency. The only segment of the market that might be interested in that combination would be very casual comfort-bike cruisers, but those bikes are typically near the lower-end of the price range for all bicycles. Using a belt would rule out an external-gear hub, and internal-gear hubs cost considerably more than externals.

So I would suspect that the only sort of people (very casual short-distance riders) who would be interested in a belt-drive bicycle wouldn't be willing to pay what an internal-gear-hub bike would cost.
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Every few weeks somebody starts yet another thread about some great "new" technology like airless tires, shaft drive, belt drive, CV drive, automatic transmission etc, etc, ad nauseum that is going to revolutionize the bike industry. The truth is all this stuff is not new and has been repeatedly tried and failed to gain any significant traction.

Belts have been used in machinery forever. It's not some great leap forward in technology. If it truly offered a significant improvement for bicycles, we would already be using it.

The modern bicycle is a design that has been continually optimized over the last century and it is very good at what it does. There may be niche markets where some of this stuff makes sense, but don't expect it to make a significant impact.

... and if man were meant to fly, god would have given us wings.

The potential is there. There is a thread up in in mechanics about efficiencies of internal hub drives which includes a link to some good testing done on various hubs versus geared systems: https://bikeforums.net/showthread.php?t=363900

If the efficiency of the belt is at or close to that of a good chain and the hub efficiency is close as well, that would make one heck of a commuter/foul weather bike. If I am not racing, I would be willing to spot 2-3 % to get that. Heck, you can get more than that back by positioning & your choice of shirts.

That's what I think too. I suspect that the issues are more commercial than technological.

Well, he sure as heck wouldn't have given us propellor beanies and an endless supply of threads about how they are going to take us to the moon.

I'd love a belt drive for my 3-speed internal hub commuter. Less greasy, interesting look, etc.

As Thor29 pointed out, getting inside the rear triangle is the biggest problem at this point in time. Belt technology surpasses the requirements, but frames have to be built to accommodate them. With so many people willing to accept the archaic derailleur system, it will be a while before belts are common. Actually, it will take as long as it takes for internal gears to take over. I feel pretty certain electric bikes will be common before that happens.

Harley switched their big twins to belts in the mid-80's and their Sportsters in the early 90's. I'd say belts are ready for prime time.

I've owned chain,belt,and shaft driven motorcycles. The belts blow the others away. The only time I've ever adjusted the tension on a belt was when I changed the rear tire. No lubing/changing oil,no cleaning,nada. And they put zero wear on the pulleys(sprockets).

A belt/IG hub combo wuold be perfect for a commuter or touring bike. If the hub could handle it,it prolly also would work for most MTB's.

Just have to have frames that would accomodate the belt. It would need either elevated chainstays or dropouts that could be unbolted from the stays to get the belt mounted.

Most folks lock onto the 2-3% energy loss over chain drive. However, this amounts to something on the order of 0.5% total loss overall, since frictional losses are only a small part of energy expenditures on a road bike.