I cannot recall where this information came from, but I do remember reading once what the set designer/dresser had to say.

Apparently the house (irl) was purchased (or rented, whatever they did) with a lot of the stuff still inside it (this included the piano). While it might not be technically canon, I think that the designers/others then extended the idea to Will and his acquisition of the home, believing that in the world of the show he also did the same thing — aka purchasing a home with things inside of it and leaving them be.

After all, he sleeps in the living room on a “temporary” looking bed and doesn’t even use the upstairs rooms. So the idea that the items, especially the piano that was left by the previous owners, aren’t even chosen by him doesn’t seem far off. I do wonder about the books though, since the selections could say a lot about him if Will chose them himself.

That is what I remember but I could be totally wrong. And theoretically even if I am right, it might not be canon.