Helmets reduce peripheral vision. I would absolutely be comfortable banning helmets in cars if people started wearing them and causing more accidents.

My bicycle helmet sits on top of my head, and has nothing to do with peripheral vision. Do you wear yours pulled down low over your forehead? If so you're doing it wrong.

The strap makes it more difficult to turn your head if it's properly tight. Probably also will hit your head on the ceiling if you're in a sedan.

I think you've been fitted into the wrong helmet. Do you really think MTB riders have any more difficulty turning their heads? It's not the case. And to back up what the other person said: modern road and XC/enduro MTB helmets do not obstruct peripheral vision unless it also has a non-retractable visor. Even modern full-face helmets limit vision less than you might think.