The ambushed vehicles will run for max a few weeks without maintenance. Then there is ammo of which they use kilotons (thousands of tons), and you obviously can't really capture such a quantity of ammo. Though you can capture some ammo, sometimes.

The captured vehicles could often be serviced by either side. Also, some vehicles could be traded with foreign partners'. I bet you there are multiple T-90 and even more interesting vehicles in Yuma Proving Ground and other compounds, disassembled to the last screw.