Not being there, I’m gonna guess that the threads are kinda rusty right where bolt goes through the frame. And the threads on the hole are/were rusty.
When turned, the rusty surfaces wiped each other smooth and they spin freely.
Now there is no grip/threads to push the fat rusty crusty part on the inside out the hole when turned with a wrench.
I have put vise grips on bolts in ^that situation so I could both turn/pull out simultaneously. The threads may bite enough again to get it out.