Best theory I've read is that sometimes Gmail needs to let some unclassified spam through and have users report it to get feedback on an algorithm change.
I don't really have one of the former, but have been pretty disgusted with the quantity of the latter for quite a while now.
My 15 year old email is finally on some spam list and somehow Norton, Macafee, Fedex, Office 365 and Loan scams can come straight into the inbox.
the only email i get a lot of spam on was in a data breach