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.
Isn’t this supposed to be the goal of NATO militaries into the Ukraine proxy? Take out as much Russian firepower as they can. They might as well pump the equipment into Ukraine while they are still fighting, because what’s the point of keeping any.