The crazy Uber drivers would replace their windows with plexiglass if that caught on.

Your threat model is incoherent.

You are assuming a crazed uber driver is smart and knowledgeable enough to do that, but 90% of people driving ubers to start with are doing so because they don't have those kind of skills or knowledge.

Well, this is a whole different kind of ignorant.

I think the crazy kidnappers might have another reason for doing it other than lacking skills.

You can just unlock the door and exit the car in that case-- there's no way to "lock someone in" unless you've modified the lock somehow. And if someone did that, they probably reinforced the glass too.

Child safety locks do this. They prevent the rear doors from being unlocked and opened from the inside.

Well, I learned something new today! I always thought that kind of thing would be illegal because it's a fire hazard -- if the driver was unconscious after an accident, the passengers could be trapped in a burning car.