e.g., AT&T could provide perfect service to the home endpoint, but the customer bought some aftermarket router from their cousin, who had configured it for Verizon. Customer calls AT&T to holler. Tier 1 support doesn't know what that particular router config GUI even looks like, so it gets bumped to T2 or T3. Ultimately to find out that the customer's cousin had hardcoded DNS to some internal Verizon system that's not visible to AT&T.
Repeat x100K. ISPs job isn't just "provide the Internet," it's also "provide all the troubleshooting for every non-technical customer who just wants to watch Netflix but doesn't even know what a router is"