Where does it say it’s voluntary?! And, again, you need to take it in line with literally every other treaty in effect, which emphasizes the ways that defense works.
It can’t be automatic because US domestic laws prohibit the president from declaring war without congressional approval, not because taking part in a defensive pact is somehow optional. And, again, sovereignty is baked into all international laws, especially those having to do with the United States (it’s always been really serious about maintaining this more or less absolutely).
You’re stating a “fact” that’s incorrect, and works in the interest of countries that would benefit a great deal from a lack of adherence to Article 5. Stop it. If any country decided to not participate when Article 5 is invoked, the alliance would end. It’s quite literally the cornerstone of the whole deal.