They are allowed to. They just are unable to - not enough troops and their economy is destroyed and they face well made fortifications and superior firepower. The NATO emperor has no clothes, no ammo and insufficient funds. How much good money do they want to throw at the lost cause?
If NATO took it from proxy war to direct warfare they could push Russia back and start ww3 with missiles in their own countries when they have depleted air defence and this scenario comes with high probability of escalation to nuclear Armageddon, billions dead. For this same end-game reason Putin wont go beyond sorting out Ukraine, he cant take on NATO directly with conventional weapons and win but he is almost certain to win a conventional war against Ukraine as we knew from the beginning.
