> Which part of these required a new protocol and couldn’t be built before @at existed? Seems to me we’re reinventing the wheel for I’m not entirely sure which benefit.

The atproto folks went and categorized all of the other attempts to do this at the time. (They even had some I hadn't heard of!)

All of them make various tradeoffs. None of them were the set of tradeoffs the team wanted. So they needed to make some new things. That's really the core of it.

My sibling has one of the largest and most specific things, but this is the underlying reason.