I think 5.5 is a generous score considering how poor the movie is.

I respectfully have to disagree with you. No suspense, cartoonish humour, cardboard characters and a lame killer with an even lamer motive.
waits for Guy to jump on me

Blame the women much!?
Upping the comedy at the expense of horror - that's a problem. Blame that on the idiots who shot up Columbine.
The main actors looking tired of going through the motions - that's a problem.
Ehren Kruger writing script pages once filming had already started, resulting in bad dialogue and characters splitting up and making stupid decisions - that's a problem.
Courteney Cox's hair and yellow pantsuit is a problem. Patrick Dempsey's dull-as cop too. The entire 'it's a trilogy' thing when the alternate endings for Scream 2 prove anything but.
The whole production looks rushed and like obligatory. It's a 7 at most.

I agree.
Perhaps the whole brother plot is why the rating is so low?
It could be Courteney Cox. Her hair. Her scream.
It also could be Jenny McCarthy.
Or the lack of Kevin Williamson.
Or the toning down of the explicit gore of the first two films.
And, as much as I like her, it could be Parker Posey's overacting.