Just logistically speaking, imagine needing to send dozens of different types of dozens of spare parts to different sections of a thousand-mile wide front line, with different types, shapes and calibers of ammunition as well. These tanks will likely just be abandoned after they get even slightly damaged.
It’s fucked in many ways. They see Ukraine as a black hole that absorbs their old stockpiles. But no, it’s not like that, it’s a warzone. These weapons are being used to kill people, they don’t just disappear into thin air.