Some reasons that crank arm keeps coming loose can happen, such as: the bolts are loose and damaged; incorrect installation; the bearing is damaged; the spindle is wiped out; An unfitting bottom bracket,...
My crank arm constantly comes loose, although I tightened it. I go to the bike repair store near my house, and they say that the bearing of the bottom bracket is worn. To fix this, I need to replace it with a new one, grease the new component, and tighten the bicycle crank.