Why do so many mass shooters have a political "manifesto"

Idk where else to post this without it being taken down. I don't understand why every single shooting is narrowed down to two political ideologies ( either alt right/ white supremacy or radical leftist/trans activists(??)) Even before the details are out, people already start blaming it on one or the other. For some reason I don't believe that this many young people are naturally being indoctrinated into political ideologies to the point where they will kill for it. Why do these kids even give a fuck about politics? The narrative of the bullied/rejected/abused/ mentally ill kid who just snaps one day seems more organic to me because it's something personal