In many European countries, there actually is a speed limit before almost every turn or intersection that's in or near a bigger city, and thus people begin to think like this, which is the point - the speed limit is so overused that people stop thinking for themselves.
The problem of speed limit overuse (recognized even by our officials) is a lot smaller in western Europe, if you got your experience from their roads.
It would be unreasonable (& annoying) to have different speed limits for every different corner/stretch of road. There's also considerations such as the weather & lighting conditions to take into account (& while I have seen signs on motorways for variable speed limits based on those in Europe, you cannot expect every case to be covered).