Many hoarders don't leave the house for fear of the outside world (and the people in it). Some don't cook because the kitchen is inaccessible, don't bathe because the bathroom is stuffed with mail, and can't park in their garage because it's filled to bursting with machine parts. Their physical existence is dictated by the things they collect.
Jumping into a discussion on the existential drawbacks of living this way is probably not going to help, at least not at the beginning. Start small, basic, and in the moment: Do you want to take a shower? Maybe we could start going through the mail in the bathroom to make some space.
The books you store in the oven, maybe we could pick a few to donate so someone else can read them and we could make dinner together?
It does get more complicated where animals are involved, because they need to be removed, and the people who hoard them typically love them and simply can't see the harm they're doing. This is one of the places where some outside help might be necessary.