Because when you admit state that has part of it's territory occupied into a defense pact, you are effectively declaring war on the occupying state.
Imo it's not quite arsehole thing to not start WWIII due to non-alliance country.
People are also forgetting that you can't just instantly admit Ukraine into NATO and instantly teleport all the alliance armies onto the battlefield. Trying to make it quicker would very likely just hasten the Russian invasion (because the head douche apparently wanted Ukraine all along), so it really boils down to "do you want to start throwing nukes for state that you never made nuke-backed promises to?"
It's appalling how many people here are not seeing bigger picture and would love to just try to end humanity just because some nation got dealt _really_ bad cards. Economically Russia is cratering, so the impatience to end it all here (or in 2014) is just irrational. They were already losing technology (e.g. see how their space program is faltering), this will just accelerate Russia's slide into irrelevance. Even their weapons tech is not keeping up, see the latest kerfuffle in Caucasus: https://en.wikipedia.org/wiki/2020_Nagorno-Karabakh_war
Also the wording is fuzzy when it comes to conventional war, but maybe the wiki has wrong wording. Either way, that is nowhere close to full defense pact which NATO is (with forward allied military bases and such).
EDIT: and wiki wording is unclear whether the treaty applies for conventional war. That's all I meant by "fuzzy"
NATO treaty itself does not talk about rapid response forces, joint bases and so on. All of that comes beyond the treaty. What stops key states from retreating back to basic treaty and then finding an excuse when Article 5 comes? E.g. if Russia comes as non-marked „rebels“?
Maybe NATO has also weak wording in the treaty itself. So ask yourself - why did Ukraine want so badly to be accepted into it? Why is Russia opposing NATO expansion so much?
Why did Baltic states or Poland wanted into NATO so much? It's one more treaty you can show to media when shit hits the fan. One more treaty your supporters can use debating governments for help.
In reality, if Russia attacks a NATO member, it won't be automatic full-on NATO response. It will still be debates. But certain people will be much more uncomfortable saying „no we won't defend NATO allies“. Yet some will still say it.
Russia wants it's own NATO, with black jack and hookers. And countries joining the real NATO obviously won't join theirs.
Baltic states are undefendable, even compared to Ukraine. If Ukraine is down, Poland is as indefendable as Ukraine. Then we are back to Cold War state where Germany is barely defendable.