The most rational reason? You don’t actually get to your destination any faster.

Autobahn
A German car club actually tests this every few years. They drive two cars from one end of the country to the other, a trip of just over 1000 km. Two drivers take identical cars for a trip, one drives as fast as the law and road conditions allow, which means 220 km/h over some stretches of the Autobahn.
He’s not allowed to break the speed limit or endanger anybody, but should drive as fast as possible. The other drives an economical pace, which works to something like 110–120 km/h for the most part.
The test was ran several times over by now and the result it always the same: the speeder gets to his destination approximately 20 minutes earlier and considerably more worn out. Yes he does race through some stretches, but the increased fuel consumption and driver fatigue mandates more and longer stops, which make up nearly all the advantage.
So really fast driving doesn’t actually benefit you a whole lot on long journeys and the added fuel consumption makes it considerably more expensive as well. It makes even less of a difference on shorter trips, because there’s no time for the higher speed to have a significant effect.
Europe is looking into a continent-wide 110 km/h speed limit for precisely that reason. Cars are very fuel effecient at that speed and higher velocities don’t translate into meaningfully shorter trips. Most Germans fully internalised this and drive effeciently.
This is a photo of my sister driving taken by me in the passenger seat on the autobahn somewhere in Germany, I think heading back to Berlin from Weimar. As you can see we were travelling at 150km/hr.

The car on the left on the outside lane or the fast lane overtook us at speed. This happened many times during our travels, mostly newer luxury type cars. They seemed to be hitting at least 160, some maybe even getting close to 180.
Yes it can be risky travelling at such high speed on the autobahn but the roads are very good, and there are many straight sections which go for ages.
Many German cars have wider gear ratios as well, say six gears inside of five in a manual car. I have a VW Golf and this is the case. Having an extra gear I would say means the car is designed to go at higher speeds safely. But this would depend on the driver I suppose, and the car. Also, some people just don’t have the confidence for such feats.
But I imagine if everybody drove at 160km/h, well they couldn’t. You’ve got trucks, vans, old cars, etc. and people merging all the time. It just wouldn’t be possible. There’s the fast lane, and most people aren’t using it unless they’re going damn quick or overtaking.