I'm not saying I agree even with this, but to fair I think the argument usually is that what they're really saying is cops as part of the system currently are bad.

ACAB is a horrible slogan if that's what they're going for because people react like you, which makes sense, but its more to get attention.

So the point is that it doesn't matter if cops as individuals are good people, the job itself and the justice system they are a part of is inherently corrupt and falling apart, doesn't work, etc. And they're part of the problem by being cops at all, because that means (to them) that these cops don't see the problems or don't see them as important enough to not be cops.