I think that has to do with the security they set up to try to ensure malware/viruses don't go upstream. If you are going to share permissions across users, across computers, and across multiple networks, you can't have Fred downloading an email to his documents which is automatically backing up to his onedrive which he shared with a sharepoint drive for everyone else to access and now it has permissions to come back down to their devices as well. I would say that's ignoring teams, but like it was pointed out elsewhere, SharePoint / OneDrive / Teams storage is essentially all the same.