Again, you've cited no source.
>Because black people don't do random violence
Clearly you are misinformed then. You seem to be operating on stereotypes and assumptions rather than data.
I like how you have changed it to "random violence," when we are clearly talking about "mass shootings." But even if we change it to non-gang-related "mass shootings" there numerous instances that speak against your conclusion. Remember though we are considering whether white people are "almost exclusively" the perpetrators of "mass shootings" (non-gang related, though it is telling you have to edit that out when black people are so disproportionately represented when it comes to general shootings, murders (approx 50%), assaults (33%), robberies (50%), and rapes (33%) when they only make up about 5% of the population).
The truth is that people downplay black mass shooters because they do not fit the media narrative about the types of shootings black people are expected to participate in. This is also why people also incorrectly assume that serial killers are exclusively white (one contributing reason is that black serial killers are likely to kill in urban centers and kill people like prostitutes who society does not care much about and expect to be killed - these killings are often assumed to be unconnected killings; see: http://www.amazon.com/gp/product/1597775754?keywords=killing%20for%20sport&qid=1444702102&ref_=sr_1_1&sr=8-1 for more).
For select examples contrary to your assumptions:
https://en.wikipedia.org/wiki/Mark_Essex (racist killer who shot at paramedics, firemen, and civilians)
https://en.wikipedia.org/wiki/James_Edward_Pough
https://en.wikipedia.org/wiki/Long_Island_Rail_Road_massacre#Shooting (they even tried to excuse his actions with a 'black rage' defense)
https://en.wikipedia.org/wiki/1993_Aurora_shooting
DC snipers random enough for you?: https://en.wikipedia.org/wiki/John_Allen_Muhammad https://en.wikipedia.org/wiki/Lee_Boyd_Malvo
https://en.wikipedia.org/wiki/Washington_Navy_Yard_shooting https://en.wikipedia.org/wiki/Hartford_Distributors_shooting https://en.wikipedia.org/wiki/Kirkwood_City_Council_shooting http://abcnews.go.com/US/story?id=94177 https://en.wikipedia.org/wiki/Hastings_Arthur_Wise http://articles.orlandosentinel.com/1996-02-10/news/9602100158_1_mccree-fort-lauderdale-trailer https://en.wikipedia.org/wiki/Appalachian_School_of_Law_shooting https://en.wikipedia.org/wiki/Christopher_Dorner_shootings_and_manhunt#Manifesto_for_killings
http://www.tampabay.com/news/education/college/fsu-shooters-friends-tried-to-get-help-for-him-months-before-the-shooting/2207514 (nearly a mass shooting - 3 people)
It seems you are denying this data because it does not fit with your preconceived notions. Unfortunately mass shootings are not the sole property of any one racial or ethnic group, and not whites more so than their population (again less, while black and Asian more)