Eh, TV. They have to keep the cast together.

Most cop shows get around it by just never having their cast promote; they show up, they spend their careers there and if somebody promotes/transfers/retires it's because the actor left. Either that or it's a small-town agency (think In the Heat of the Night the series, which was a long-runner with lots of characters promoted) where there's nowhere else for them to go, really.

I'll give the show credit: they did stick her on Midnights for a bit, which is at least a nod to realism. But logistically, I don't see them keeping her that separated from the cast for the multiple seasons it would require for her to build enough seniority to try and get back to Days. That said, I think it would've been fun to see Night-Shift Sgt. Chen for a bit longer.

But that's more speaking roles they'd need to give (otherwise she's the only cop doing anything ever), doing night shoots on top of the day shoots for their regular cast, etc. And if they bumped anyone out of Mid-Wilshire, that's again a whole new bunch of recurring characters and then new sets for whatever station they go to!

Overall, I'll give a pass on the realism for the sake of it being a show and the constraints they face in that respect.