Django releases a new version every six months. Packages that aren't part of Django core are free to release on a different schedule, which really helps when you're trying to innovate on something new.
In DRF's case it's now mature and stable enough that maybe that wouldn't be so much of a problem... but it's still not clear to me that having it in Django core would be a net benefit (aside from as a marketing thing) than having it live independently as it does today.
DRF’s validators don’t implement `__eq__`, Serializers doesn’t implement `serializer._meta.fields` on the base class (you have to instantiate the serializer to access anything other than `declared_fields`), lack of async view support, and on and on.
I find DRF grating and would pay anything to have an official Django REST api contrib package.