Well, the problem for them is that it's small numbers, 10-20-50 western tanks can make absolutely no difference, especially if they are manned by hastily trained CIAkrainian crews. If they sent them 500 pieces with all western crews, that would be something, but the deaths of a large number of Brits/French/Germans would have a rather negative impact in those countries.
This war has only two potential outcomes:
1. Without the direct participation of NATO, above all hamerica, Russia wins in the long term. The more weapons are delivered, the more destruction there will be in Ukraine, the more people will die, but it will not affect the outcome;
2. If NATO flies in directly against Russia, it will not be able to defeat the entire "collective West" by conventional means, but then a nuclear exchange is the most likely, because for Russia the continued existence of the world without Russia is meaningless, as has been clearly stated many times.
Since for Russia this is a war of necessity and an existential issue, and for the "west" it is not, and I am not sure that hamerican neocon criminals are willing to die, the first outcome is far more likely.