For the same reason Cuba joining the Warsaw Pact wasn't even a thought during the Cold War. The USSR was very unlikely to attack the much more powerful US but despite that, having a rival military alliance is going to be seen as a threat.
It really comes down to applying standards equally.
The "why" is irrelevant. The US wouldn't give a fuck.
There's no scenario where the US would say "Oh ok, that's a great reason to join up with China. Yeah, go ahead set up a bunch of bases with missiles pointed at us."
Great powers don't behave that way and never have.
Most experts point to the 2008 Bucharest Summit as the start of it all
https://en.wikipedia.org/wiki/Ukraine–NATO_relations#Bucharest_summit
Extremely unlikely that Ukraine gets annexed and "ceases to exist."
It's one thing to defeat a country, another to occupy it, and another to annex it. As both the US and the Soviet Union learned, occupying weak, Middle Eastern/Central Asian countries is extremely hard. So there's just no way that a much weaker Russia today is going to annex a much more militarily powerful country like Ukraine with its 30m people who will be understandably hostile to it.
Hitler's Germany is much, much, much more powerful than Putin's Russia.