|"It's usually somewhere in the documents anyway (1003, W9, etc)."|
Seller packages don't have a 1003 and might not have a w-9, depending on the circumstances. Also, especially where a home is being sold by a son or daughter because a parent has died, the signer hasn't had to have any dealings with a lender nor have they had to provide any of the personal info that a buyer would have to. Some folks (including me) automatically hesitate or refuse when asked to provide a SSN, so they may be caught off guard with some of those very annoying tax forms.
I can't recall ever having this issue with loan docs, which, as you said, nearly always (except for the rare W-9 or 4506T for a non-borrowing spouse) already have the SSN pre-filled.