De jure, it's because NATO is a defensive pact. In other words, they didn't help the United Kingdom during the Falklands/Malvinas conflict or France during the First Indochina War and the Algerian conflict.
De facto, it's because they fear that Russia could retaliate by using nuclear weapons or intercontinental missiles that could easily hit the US. What's more, these troops would be of little use and would actually turn European public opinion against the war. And they would be used as propaganda for Russia to inspire Ukrainians to desert their army to fight a foreign enemy.