I get it, it's the only experience you've known, so it must be right.

[USA] https://en.wikipedia.org/wiki/National_Maximum_Speed_Law

Meanwhile, your whole interstate system was inspired by the German autobahn. Look at an interstate circling a city. Most of them are 65mph, often with minimum 12ft lane width, awkward turns skirting district borders, interchanges every mile or more, and persistent dense traffic. Then look at your middle states with incredibly straight interstates, significantly wider 20ft lanes, miles between exits, no commuter traffic. It's still 65mph except for some brave areas that increased it a whopping 5-15mph in the early 2000s. Absolutely no one in that engineering company concluded 65 was the maximum safe number. It wasn't 80, either. The average car has become far, far more capable and stable than those present in 1974. Try one of those on a current 65mph highway some time.