Because the API is still evolving, and it's also useful that it supports other homeservers than Synapse. It's also written by an entirely different bunch of people (it's effectively written by the Dendrite team rather than the Synapse team).

Once the API is stable I'm sure all the homeservers will implement it natively. Until then, think of it like a reverse-proxy that lets an HTTP/1 webapp talk HTTP/2 to the outside world.

> it's effectively written by the Dendrite team rather than the Synapse team

I would then ask why it isn’t integrated into Dendrite

...because then Synapse wouldn't be able to use it?

I would guess it's so the same functionality can be used with other Matrix servers without having to rewrite the whole thing

It's currently still a proposal to the spec, and it is not yet part of the Matrix protocol.