Haughtney - I can't honestly say I know why, but I am yet to hear someone with authority say I have completely got this wrong.
As far as manouevring as you like, I can not imagine that at any reasonably busy airport you are going to be too far from established on final at 5nm anyway.
I can assure you however that carte blanche manouevring at 5nm from a base position at the airports I am familiar with, can (and does) lead to a technical breakdown of separation standards with either departure paths or missed approach / final approach of converging runways - so that is one answer for you as to why I guess.
Frankly I am not arguing as to whether it is a good idea or not - just trying to make sure we are all dancing to the same tune.