It's not that I think collecting things is strange in itself. It's when people publicly display evidence of wildly age-inappropriate interests or decorate with heavily branded stuff (a lot of which could be considered advertising) that I start to question why they doing it.
But you're actually right: I don't collect anything for the sake of keeping a collection. I buy books, but I only keep them if I want to reread them and they get doubled up on the shelves because those are for storage, not for showing off. Same goes for other physical media. If I put up art, it's something I bought individually because it looked good rather than merch from a franchise I like. I hate clutter, so that might be part of it. It might well be boring for visitors to look at, though.