Actually why can any car go that fast?

German car, Autobahn. But most modern cars can go that fast. Petrol/gas usually faster than comparable diesel.

The high top speed is a by-product of the amount of power cars now have, power they need to accelerate from 0-60 fast enough for modern traffic, and to comfortably and safely maintain motorway speeds. Large powerful engine which never has to go high into the revs during everyday traffic, is likely also more efficient, but I may be mistaken.

Cars which can’t reach speeds of over 100mph, are really slow to accelerate, and aren’t comfortable, reliable, or safe at motorway speeds.

Eg. depending on the model a Lada Niva, will have a top speed of only 120kph/75mph, but you really wouldn’t want to drive over 100kmh/60mph for extended periods. It’s not reliable, comfortable, safe, and I suspect it isn’t efficient either.

Now obviously, you could limit the top speed, but that’s a matter of regulation, and relatively easy to bypass.