Because Amber isn't the perfect victim, and patriarchy has brainwashed women into thinking they can protect themselves from abuse by being perfect, and that women who aren't perfect victims are to blame for their own abuse.
The fact Depp is violent and abusive really is not in dispute at all. Look at the history and evidence:
He had to give his first wife a huge payoff after leaving her a verbally and racially abusive (use of the N word) voicemail.
Has a long history of beating people up, including photographers, security guards, hotel staff, and movie crew members.
Once set fire to his own costume to protest against something he was upset about on set.
Bragged to a magazine interviewer that it was worth going to jail "to see the look of fear on the little maggot's face" (referring to beating up a photographer).
During a TV interview, Letterman asked if it was true he beat people up, and he laughed and said "yeah sometimes people have it coming" and also laughed about trashing hotel rooms.
In a different Letterman interview many years later, laughed about being arrested for assault after he walked down the street swinging a 4x4 plank of wood.
In a third Letterman interview yet more years later, made a comment about flying to Australia to beat the shit out of someone.
Ex-girlfriend Jennifer Grey wrote in her memoir that he was constantly getting into physical fights during the time she dated him.
Ex-girlfriend Ellen Barkin testified in court that he'd thrown a wine bottle at her head, and described him as very controlling and jealous.
Has made dozens of alarming and violent comments in various interviews over the decade, including telling Playboy magazine that he suffers from what he calls "hillbilly rages" where the red mist descends and he doesn't know what he's doing, a different interview in GQ where he discussed the fact his favourite thing to do during physical fights is to bite people, and various interviews where he discussed his serious substance abuse which started as a minor.
Smashed up hotel rooms with women he was dating locked inside them.
Beat people up in front of women he was dating.
Made the following joke in a Rolling Stone magazine: "Hunter Thompson zeroes in on faults and good points immediately. I was with Kate [Moss], and I think he went straight for the romance jugular, shit like whether I beat her enough. I probably told him, ‘Yeah, she gets a severe beating.'”
Told interviewers that Vanessa Paradis knew how to "talk him down from the ledge" when he experienced his rage episodes (meaning at the very least that she had to do emotional labour of managing his anger).
Vanessa Paradis said in interviews that he used to explode and throw plates at walls.
Paid his ex-girlfriend $150 million dollars despite them never having been married, then sent texts claiming she was "an extortionist." Hmm wonder what a man with a known history of paying off women he's subjected to violence could possibly be referring to by giving a woman $150 million then claiming she extorted him.
Paid a huge settlement to a respected middle aged disabled female university professor, after he had his bodyguards assault her, illegally handcuff her, and drag her across the floor so violently that part of her clothing was torn off leaving her nude from the waist down, and her shoulders became dislocated. Because he objected to a disabled person being in the VIP area of a concert.
Sent texts referring to the mother of his children as "a withering cunt".
Sent dozens of explicitly and extremely violent texts, including texts talking about raping and murdering his wife.
Admitted on video to having beaten his wife.
There's audio of him admitting to have headbutted his wife.
The London High Court listened to evidence relating to 15 separate incidences of domestic violence (some of that evidence was given privately and never shared with the press or public). Of those 14 incidents, the High Court found that the evidence proved that Depp had beaten up his wife on 12 separate occasions, that on three occasions she'd been made to fear for her life, and that for the other three alleged incidents the evidence was not sufficient to prove assault.
The London High Court ruled that as the evidence proved that Depp had beaten up his wife on at least 12 separate occasions it was factually true to describe him as "a wife beater."
Whatever Amber did or did not do, it's pretty clear that Depp is a violent, unstable, abusive man and has been for a very long time.