That and they treated plate armor like they do modern tanks, in that certain sections of it are far more armored than other sections. The most armored part of full plate armor was the torso. The rest of the armor was relatively thin. There was this one Youtube video of a guy in full kit running, jumping and even falling off of his horse, yet he still managed to jump to his feet in an instant. 

And it didn't need to be especially thick or heavy in order to do its job, which was to stop piercing and slashing attack from swords.

Warhammers, however, pretty much ignored the protective qualities of plate armor.

I do have this odd feeling that post people in Arena RPs perceive plate armor as being completely invulnerable to melee attacks or something.