I always felt that when monsters dropped armor and weapons, they got them from the adventurers they ate...

"Hey Vaan. Is the a sword sticking out of its stomach?"
"Yeah, I think it is."
"Pull it out and maybe we can use it."
"......why do I have to do it?"
"Cause I'm the 'leading man' and you're a little bitch street orphan so get your pedo-bait ass over there and grab that sword"
"Okay okay... geez..."