I used to think the same. Recently, I was surprised to learn that many US retail brokerage firms now offer this service.

If you Google (or DuckDuck) "fractional share ownership", you will find a bunch of hits with a list of brokerages and an explanation about how it works.

I assume these products are "captive audience" -- you must buy or sell your fractions with the same broker. Thus, they cannot be transferred to other brokers. That said, I also assume it a very price competitive product so the margins would be thin and prices fair.