Austria is an odd example, given that it's completely surrounded by "friendlies". What would the Austrians gain by joining NATO? It's fairly clear what they would lose, which is the ability to stay out of a future NATO conflict.
Thought experiment: let's imagine the UK or the US were to somehow kick off a hot war with - let's say - Chinese forces, on/around the Spratly Islands in the South China Sea.
Do any of us seriously believe that the UK or the US just press some magic "Article 5" button and troops from all over NATO just grab their weapons and set off round the world to join in the fighting?
Looking back at the history of NATO's Article 5 it's clear it has more holes than a Swiss cheese. It's political theatre.[0]