Because placing a 4 minute objective in a base that takes 3 minutes to capture is rather dumb I mean, you could just place them in every base then anyway.
The point is the SCU is so close to the spawn room and have such clear line of sight that if you can succesfuly overload it and defend it then it is very likely that the defenders are already being held back and that the points have already been captured, thus the scu wont even have time to explode.
not necessarily, in my experience with latticed biolabs often only have half the points capped before the shield goes down, I've done many successful generator pushes as a defender since the cap time change, it just now forces defenders to be less passive.