The speed limit isn't a suggested speed, it's an absolute maximum (excluding motorways with a minimum of 60km/h). If the road is frozen over you can't drive the speed limit either, the same applies when it's slippery due to rain or leaves or when the lights are off.
You always need to be able to react to sudden movement, no matter if it's a pedestrian crossing the street, a motorist leaving their own driveway or even a trash can rolling into the road. It should be in your own best interest to avoid accidents.
The entitled attitude you ascribe to the overtaking drivers but also display yourself is just going to cause problems for everyone. Trying to shave a few seconds off of your commute by speeding in dark areas isn't going to get you home any faster, all you're doing is increasing your own stress level and risking someone's life.
A little bit of respect on the road would go a long way to improve everyone's experience on the road.
If you actually think about it, it's absolutely makes sense. The Autobahn has additional stopping lanes for broken down cars and several meters of grass to each side, which means you can safely drive hundreds of kilometers an hour while still being able to see obstructions early enough to brake in time.
Slower motorways have smaller setbacks, but still enough to keep their speeds.
City streets where you can't see people entering the road in time to brake usually have relatively low speed limits to reduce the braking distance as well as the damage caused by a collision.
But if the visibility or braking distance are affected due to weather or broken streetlamps, it's up to you to slow down accordingly. But even for situations like that traffic planners usually add additional signs, it's common to see roads with signs that say
to warn people to drive slower when the road is freezing or signs that say
to warn of crossing animals in the next 400m and set a lower speed limit.
The same obviously applies when it's not crossing deer but crossing pedestrians.