Cos the politicians and road safety camp have all collectively bought into the Speed Kills (even although their own accidents statistics do not bear this out)and its bad for you to have fun on the road messages. Also speed = increased fuel consumption = bad for the environment.
Its also considerably easier and cheaper for them to target speed and introduce speed reduction measures than address other causes of accidents such as :
potholes and badly maintained road surfaces,
poor road design including reduced sight lines caused by lack of vegetation pruning,
poor road signage and lines,
poor driving standards,
apparently non-existent police traffic divisions to enforce better standards,
lack of winter snow clearing and gritting, etc.
I predict in 10 years that we will probably have speed limits reduced to such an extent that a 1930's vehicle would be wholly suitable in terms of engine performance and speed.
Also would only need two gears on your BMW, cos you would not be allowed to go fast enough to justify getting into 3rd gear anywhere in the UK without the mandatory little black box recorder limiting your speed.