we rarely get clearances to enter
Then even more so the reason not to raise the floor!
If one can't get a clearance, it would mean it is unsafe for more aircraft to operate inside it. (ie. too much traffic)
By raising the floor, there's a risk of further compacting aircraft inside it into tighter spaces, as well as allowing even more aircraft to fly beneath it without adequate separation. Now factor in the amount of airspace busts and you get an unsafe environment on both sides of the fence.