I'm from Europe, so probably that's my surprise. Most of the cars around here are still (I hope we will never reach what's in the US) mostly city-compact cars (VW golf size) so visibility is quite OK and rear mirror + sensor is more than enough.
(though, I'm now in Chile and amount of huge cars/SUVs/trucks/pick-ups on the street is simply mind-boggling... whhhhyyyy on earth one would drive this monstrosity in the city?!)
The manufacturers did not want to have to build these into every car, they wanted to keep charging a premium for it.