Why are speed limits less in built up areas then they are on motorways?

Why are speed limits less in built up areas then they are on motorways?
This is based on risk.
In North America we call these roads Limited Access Freeways
No pedestrians are allowed
Vehicles are expected to maintain a minimum speed.
Vehicles will only be entering the roadway at marked places and merging in a safe manner, we hope.
All these things make it much safer on the Motorways than in built up areas. So we are expected to drive slower where the risk of collision and damage is much higher.