jellicoDace
That's because that point has enough angles as it is. Adding the capability to stand on that rooftop would make it a nightmare to clear because you'd always have to expose yourself to multiple angles instead of peeking them one by one.
At least that's what I think. When designing a map it is common practice to avoid that an attacker angle can be secured by too many defending angles. There are exceptions though.
The same holds true for the bomb defending at that point btw. Bomb defenders could get just get rushed if the roof was accessible.
