Because this isn't fixing continuity at all, it's worsening it; if Fox just retcons whatever story it likes then it'll pay even less attention to continuity issues because it knows it can just nix conflicting ones.
You say this as if it's anything new. They released comics in 1988 - they were made redundant in 1992. They made more comics after that. They were made redundant in 1997. More video games and novels came along that ignored Resurrection and Ripley's sacrifice in Alien3.
They're now simplifying eveyrthing down to 5 films and the new batch of comics, novels and video games. The next Prometheus film could come out and bugger things up again.
I appreciate your disappointment that this elaborate fantasy you concocted where everything with an Alien logo on it was somehow canonical, has been all for naught, but Fox owns the thing and they call the shots.
Ditto Disney with Star Wars. I read a massive amount of EU stuff for a long time, but I think the prequels made a lot of it redundant. Their status in continuity doesn't stop some of them being good stories. I still need to catch up on Fate OF the JEdi and all the stuff that came after, and mean to at some point. Nevermind their canon status.