Ummm, because she's a self-centered, entitled asshole who also happens to be the author's pet?
She pretends to be unconscious for hours - knowing her best friend is tearing herself apart! - just to get a jump scare. She constantly flouts social norms - norms she'll helpfully point out just to remind you she is aware of them - because she's far too special to be bound by politeness or respect. She damn near derails Mallory's path by tricking Mal into one of the most awkward sleep fondling sessions I've seen, then helps her self to an uninvited peepshow if it somehow leads to sex. She stalks and sexually harasses the MC constantly, and we aren't even allowed to frown at her, much less tell her to fuck off. And of course she's the one who has to step in and save the big dumb MC from the Prizer conspiracy, because only her super-savant-brain could possibly execute a scheme as diabolically clever as "confront the stooge with evidence and demand he spill the beans" - and that's assuming you talk her out of the assault and battery felicemastronzo mentioned.
I know Amanda is meant to be goofy, sexy fun, but I just find her insufferable. The game would be so much better off without the time wasted on her endless bullshit.