Oversight is not in the core software, to start with. This is. Your question could be rephrased: why doesn't oversight use the built-in deletion interface? Maybe sometime it will, once there *is* a built-in deletion interface for single revisions. Probably it would be nicer, once rev_deleted is in, to just have a checkbox on the "delete revision" page for those with the appropriate privileges.
Essentially, you're asking about interface/naming details. It's the same feature either way.
The big difference between oversight and deletion is who can do it and who can see the revision afterwards. Single revision deletion cannot replace oversight unless there is a way to stop regular admins seeing the deleted edit. (Such a way could be implemented, of course, I don't know if Aaron is intending to do so.)
Deleting individual revisions has all kinds of other problems too (there is a discussion on wikien-l), for example if Admin A deletes one bad revision, Admin B then deletes the whole page and Admin C undeletes it, the bad revision comes back.
Only if deletion of a single revision is not distinguished somehow in the database from deletion of an entire page. I don't know how this is handled in the current rev_deleted draft, you'd probably want to ask Aaron.
That would require a substantial change to the undeletion interface. That's not necessarily a bad thing, though. Knowing which deletion event deleted which revisions would be quite handy.