I have nothing to add to the discussion about Prometheus, having never seen it, but the OP's original question as to why a sequel needs to be made, or why studios are so ready to make sequels to anything marginally successful, is simply because they're less risky.
I'd say, the Blade Runner sequel that Ridley is threatening to make is a downright risk. OTOH, teens might wanna see it because it's from the director who brought them Gladiator and 'P R O M E T H E U S'.
Apparently, Scott wants to make it because, according to him, a sequel is inevitable and he doesn't want anyone else to do it. Most reactions to the prospect of a BR sequel are negative.
A perfectly normal, regular female scientist suffers the pain of having an alien beast growing inside of her, has a machine slice open her stomach, pulls the alien out, then staples (!) her shut (that's major surgery!) and she just gets up... and runs around... and zips up a spacesuit... and acts pretty much like nothing's up...
I'd call that beyond stupid. Awful script. Awful.
I agree. There's a difference between suspension of disbelief and implausibility. The writing today is just flat out lazy, I can write what ever sh*t I feel like, and ppl are going to buy it. It's ridiculous. Or to put it another way "B" list story treatments get "A" list budgets.