You ever tried to pull over an entire freeway full of cars at once?

Throw people in jail for driving too fast? That seems pretty extreme.

It's hard to police speeding on a highway. Typically if you do get pulled over for speeding, it's because you're going way faster than those around you. So even in a 55, if everyone's doing 80, you've gotta be doing 95 or more for a cop to single you out and arrest you. (Or maybe the cop has a quota to meet.) And where I live, local cops can't use radar, so it's hard to prove how fast you were going.

And then if you do get arrested, you're most likely to get a ticket and points on your license. Get enough points and they'll take away your license, but that means you've been caught repeatedly. And points expire eventually, plus if you go to your court date and plead not guilty, a lot of the time the judge will just remove the points. So a speeding ticket from years ago generally won't have any bearing on your life except for the cost of the ticket.

So within reason, you can pretty much speed all the time consequence-free ¯\_(ツ)_/¯

Because the road design at fault, not people.

Make roads narrower, and people will drive slower

If everyone is driving that fast and there aren't accidents happening consistently then the posted limit does not reflect the reality of the road.