Because that's how we are stereotyped in America whenever some stupid shyt like that happen. You can't dismiss it because it has real consequences on us. That's the fukked up thing about being a despised minority in this country: cacs lump nikkas together by the acts of individuals. Our humanity in America is still defined as a group, not thru individualism. That's the essence of white privilege. Denying this reality would be crazy.
White folks never get this treatment even when they do nasty shyt repeatedly (child molesters, serial killers, etc).
When that wacko killed 9 blacks in South Carolina he wasn't seen as a reflection of white America, we didn't hear calls for "leaders of the white community" to speak up, etc.
Hip hop has been identified with black America more closely than any other genre in pop culture over the last 30 years. You bet that this shyt would affect how our community is perceived.
Because those are actions on entirely different sides of the spectrum and there's no innate racial animosity to point towards them or system to do it through with white supremacy.
What you just listed about white people are stereotypes, there are a lot of white serial killers, but cops or more likely to care about looking for white victims so they're more likely to get caught then black victims and if a white person targets black or poor people like they do a lot the resources might not get poured into it so if there's a black serial killer killing black people in a poor area they might not get caught or they may have an avenue to do it through like gang violence (like that crazy chick in Chicago who collected the newspapers of the people she killed, for got her name but she was some goon), i've read up on a lot of serial killers and there's a decent amount of non white serial killers but they have a higher population in this country as well and the same may apply for the other crimes you listed such as child molestation which while people on this site want to act like black people don't do that's not the truth, but the general stereotype accepted by all people is that it's a white thing and the same goes for mass shootings (which are mainly perpetuated by white people).
Plus the stuff you listed are seen as out there things only a lunatic would do while gang violence is more grounded in reality, so while those stereotypes do follow white people it doesn't harm or have the same real world effect that stereotypes about black people have on us, honestly they're seen more as a joke.
Because they're the dominant society they don't have to call for leaders to speak up, they haven't had a history in this country where they would need leaders say in the same vein that black people or muslims would and being white they're going to empathize with the criminals more even if it's subconcious and won't demonize them in the same way they would an outsider nor would they have a reason to.