Well for one it becasue of the stubbornes, for other its becasue the mentor ages super slow.
Batman we know has had dick grayson since he was at least 9. Dick is 23-24, He is aging, and in time the mantle will have to go to someone else, unless he takes a lazarus dip.
Wonder Woman doent age like normal humans, we may never see her step down from her mantle unless she is slain
The same for clark.
Whereas aquaman has a normal lifespan and has already passed on his mantle
Hal jordan has gray hair now, and we have 2 other lanterns from earth working
Perhaps Babs will come back to being batgirl, i personally hope not, DC may not allow weissman to pass the batgirl torch, mabey they want someone else to be batgirl like tiffany or onyx.
Its honestly better to have your own identity, but we know with the batgirl mantle comes a level of status and frequency of use you wont likey get as jsut another female vigilante.
We dont get enough about cass to really comment on her status, hopefully this arc will shed some light on her, but as I mentioned previous we have alot of players, and only 3 more episodes and the ventral focus is meant for Artemis and Cheshire.
I just realized Artemis was partially given Cass's backstory, particularly her time with David who is more or less this version sport master's inspiration. Luckily Artemis was allowed to develop a personality growing up.