To clarify a bit: ip banning is one of the only real tools we have to protect the site, but the entire rotating IP system is an opaque box of anonymity. There is no easy way to confirm that an IP is not shared by more than one person, or that it will rotate out to someone else's computer in a few weeks.
The only counter for this is to either not use them, or let them expire after X days. Both of which leave the site more vulnerable.
The reason ISPs do this is to actively prevent decentralization. A rotating IP really screws with people trying to self host. So you have to pay extra to get a static IP, and then they start monitoring your traffic to see if you are hosting, and want you to pay extra again for a hosting package, which they also want you to be a registered business for, so that the government can tax you more.
Otherwise self-hosting would have naturally become the means of the average user, with simple and easy to use software developed for it, and the internet would be a more open and decentralized platform.