1. Ukraine’s losses of tanks and artillery pieces are irreplaceable — as it takes close to a year to train, each crew. Ukraine is getting small arms and ammunition, not entire companies of armor and artillery. It cannot be replaced, not anywhere in the near future, and they cannot train artillerymen or tankers in a few days or weeks. Once heavy equipment is destroyed/crew killed it's *gone*. To fight as a tank company, needs training and air superiority.
(a) In a defensive battle, 1 defender can fight 5 attackers in complex terrain, like a city. 5,000 Ukrainian troops in a city needs 25,000 Russian troops to clear.
(b) In offensive action, you need to slightly out number the enemy — preferable to out number defenders 3 to 1.
2. The Ukrainian forces have tanks and they do employ them carefully in numerous delay battles. These are typically hit and run. If Russia can secure their rear area, replace lost units, and rebuild their logistics force, they will be able to restart their large offensives. The goal is to bleed the Russians and keep hitting the fuel and ammo convoys to avoid head-on fights (except where terrain favours).
3. If the Ukrainian army tries for a risky counter-offensive push with 130 or more vehicles (with 4,000 troops), the Russians will rain artillery on them. It’s not wise to risk so much combat power that can be wiped out in a single battle. Ukraine needs time and they are trading space for time.
(a) This is not a computer game, you cannot respawn to start the game again. You can see videos of Russian troop concentrations being hit by Ukraine’s artillery. You concentrate, when you have an advantage and you want to press the advantage.
(b) Currently, Ukraine is not strong enough to fight in an offensive action at battalion strength (42 or more tanks) or brigade strength (130 or more armoured vehicles). They have suffered attrition to fight the Russians. A risky counter offensive, is not worth the effort partly because these valuable trained Ukrainian troops can be destroyed by the Russians, when they concentrate for a large push.