That only works if the document is still shared with you. If the spammers are halfway clever, they're removing access immediately after granting it, so the victim can't actually open the document to report it as abusive.
Users who actually click links in the document are clustered towards early openers? Share doc for two minutes, collect clicks, unshare to cut off spam reports.
I received a lot of spam share notifications on Google Drive, and there is no spam in the Shared with Me view in the app. I don't know what their game is, but this is happening. It also doesn't make sense that Google Drive doesn't remove the notification after the file is no longer shared with me.