Why do sunvisors in cars fold down, not slide?

The sunvisors in every car in which I’ve ridden fold down. Why is this? A half-folded down sunvisor is dangerous in a crash, and a fully folded down sunvisor significantly restricts the vision of people as tall as myself.

Wouldn’t it be better to have them slide down, so that while blocking out the sun, you can maintain maximum vision?

I can answer this in one character:
$

If they slid down instead of folding down, how would you turn them 90º to shade the side window?

(And as beowulff said, they’re cheaper.)

here is an very odd one then.

In high school I helped a guy restore old cars. We worked on a car (I want to say a 41 torpedo but my memory is very foggy) The sun visor did flip down, but instead of swinging on a post to cover the door glass, it slid in a channel. I thought it was a Brilliant idea, since I always seem to find roads where the sun is blinding and in the gap the sun shade does not cover near the post.