Because prison is supposed to be about rehabilitation and not punishment. If you ban concurrent sentences, you might as well just shoot anyone with 50+ years of consecutive time in the head, it's a whole lot more humane.

It makes some sense when multiple crimes were committed but it was one event. Robbing a liquor store becomes several different crimes with their own max sentences. Sure there were multiple laws broken but they only did one thing. The issue is how grey that line becomes and how much authority a judge has over a crime's punishment and when you factor in things like if the robber shot the clerk.