I am (usually) not willing to assume that the founders of highly technical startups would not consider something that I as an outsider would in the first 5 minutes of engaging with the topic.

That makes me skeptical of all of these (minus the wind currents in cities, that might have taken a little longer).

If a startup were able to truly solve the first two issues alone, they would not be burning those world-changing engineering solutions on flying taxis.

I don't know if a silent, fail-safe, and efficient method of flight is physically impossible or not, but I do know this is low on the list of applications it would be first seen in.

EDIT: I'm looking at the air taxi companies this thread started with, and no, they have not solved any of the relevant problems.

Founders can be chasing a dream and in doing so mesmerize investors. Or they capitalize on that same dream being the investor's. Even if it's not viable, it can still be really fun company to work for and/or earn money at. Even if there is a small lane for that sort of flying machine, the sheer number of companies purportedly working on something like that is suspect. Given the huge costs for development and certification, and the small number of vehicles that will really get deployed (certainly for the first so many years), there must be many that are never going to make their money back. I worked for a drone-adjacent company and now my LinkedIn is swamped with these startups.

Theranos was famously founded on pitches about blood testing from finger pricks that literally any phlebotomist and many people with a modest life science background could've told you were physically and statistically impossible on their face. You should be considerably less credulous toward startup grifters.