The US & the UK signed the Budapest memorandum promising security assurances against threats or force against territorial integrity or political independence of Ukraine, twice now they have both failed to honour that agreement when Russia annexed Crimea and now with the full on invasion, they are legally bound to come to Ukraine's aid NATO or not.
They hide behind the "NATO is a defensive force" excuse while completely ignoring that they should be helping Ukraine as individual countries and not as the NATO alliance, they're just weak leaders scared to act