Pulling out spent units and replacing them with fresh troops would undermine Putin's efforts to control how Russians view what is going on.
Troops on R&R will talk, saying things from first-hand experience, not the official Party Line. Fresh troops would just widen the pool of people with first-hand experience that what's being put out from the government and pro-Putin outlets doesn't match reality.
So, to minimize leaks, the troops already on the shit end of the stick just get recycled. No matter how much that increases their chances of being turned into compost.
Some of the comments repeat the "he's holding his best back" speculation, but that's bullshit. The whole goal was to swoop in, take Kyiv, and set up a puppet before anyone could react. Elite units like the VDV were part of it. There was a whole "victory lap" article on a pro-Putin outlet that had been pre-written and queued to publish after the expected fait accompli (only to accidentally publish even though things were not going to plan).