Flying cars. We were supposed to have them 22 years ago and now they're saying that I should get a bike to go to work.
Ebikes are incredible. They are the most efficient form of transportation there is, they don't pollute, they are speedy, and they are compact!
- No weather protection
- Don't work (well, if at all) in snow, ice, heavy rains.
- Limited (or zero) safety features, despite going fast enough to cause injuries and using the same infrastructure as cars
- Limited speed and range
- 2d congestion limits throughput
And those "millions" of reasons all boil down to R&D costs. Reliability, inspections, takeoff-to-landing autopilots (they exist - have existed for over a decade - for military helicopter drones), etc. are all cost issues which are slowly being worked on.
The progress could be faster if it didn't rely on angel and startup-style investments and the associated NDAs and trade secrets, but I don't expect that to happen in today's political environment.
Most drivers (in the US, at least) have trouble enough navigating two dimensions. Adding a third would undoubtedly make matters worse. Hell, just look at the safety record of private jets.