Does that really take 100 years?

Feels like humans could do that in 10.

And they do that for while gaining 0 XP?

In my head canon the people are sometimes just rusty and forgetful, because even if someone lives 100 year, that doesn't mean that they will remember everything or get better at stuff. That is why I think immortal beings that live aeons, can still be surprised or tricked. Being old doesn't necessarily mean that people are wiser.

Sometimes people just need the right motivation to improve themselves.