There are many logical reasons as to why most males are mass killers, but not all mass killers are white males. Around 63% of mass shootings since 1982 were committed by white males.
The perception is inflated by the fact that the white american population is much larger than any one minority population, in addition to other factors - racial motivation and mental health.
Media and reported crime data also inflate statistics. Mass shooting statistics also include gang violence where 3-4, or more victims are involved. This includes gang altercations where both sides trade fire in a dispute, or such as a drive-by in which 3 people are injured.
Men are also more likely to own, be trained to operate, and be comfortable around guns.
Schizophrenia is most prevalent in the ages of the perpetrators - teens to late 20's, and so is paranoid schizophrenia, a more volatile version of the illness. Almost all perpetrators that were not involved in gang shootings were on antidepressants.
There's also this thing called 'socially constructed masculinity' that prevents males from seeking help when something is wrong, lest they be seen as weak by peers or families. Bullied children and teens will often not seek help and keep it to themselves. Men have much poorer support systems than women.
The explanation also lies with the notion of "the higher you are, the farther you fall" - losses or blows to ego when someone has the triple privilege of white, rich, and male are felt substantially more shameful than someone who has only one of those privileges. Some of the articles I'm reading to fuel this post were very race charged - stating that extreme racism is most prevalent and intense within white males,
*TL/DR: Not all mass shooters are white males, mass shooting statistics are inflated, and there are many logical reasons for why it is more common for white males to commit mass killings. Any number of these these issues, especially untreated or under-treated mental illness, create the formula for someone who commits mass killings. *