Do you think that you just leave a tank there and then, after 20 years, you turn the key and it works?
Yes, this is exactly how it works, a mothballed tank is taken from the warehouse, the re-mothballing procedure is carried out and the tank is ready to fight. Ukrainian fighters of the defense are similarly armed with Soviet-made Kalashnikovs, they are new in factory lubrication, as part of the Ukrainian legacy from the USSR. Old Soviet tanks played an important role in the military doctrine of the USSR, providing the ability to strike with a mechanized tank column within 48 hours anywhere in the Eurasian continent. The current military doctrine of Russia is more progressive and less relies on the brute force of tank divisions, but the tanks themselves have not gone away. That's where they came in handy.
I see... you seem to be lacking a few basic on maintenance, material sciences and mechanical engineering as Putin seems to be lacking trained crews and supplies. I am glad to know that Putin's strategic defence is based on these premises, I feel much safer. I hope the US military does not find out about this battleplan, most of them are much more aggressive than me.
Now, about that doctrine and theories... where are those tanks? They do not seem to be in Kyiv? When there is a difference between words and facts, I tend to look at facts, but that's just me.
I think these tanks for the most part continue to be stored in hangars, because in such quantities they are simply not needed in the operation in Ukraine. The very concept of warfare has changed a lot in 30-50 years. A key factor for the success of the operation in Ukraine is Russia's control over the airspace. You don't need a lot of tanks if your recon drones see the picture of what is happening, and the Ukrainian army is actually fighting blindly.