Because there’s nothing to be gained and everything to be lost from the US leaving NATO.

Yes, there are good reasons to develop an EU army with the same capabilities as the US but realistically speaking that’s decades away if we could start today but we’re not even discussing it seriously. In the meantime making any decision furthering the US from NATO gives no advantage to Europe.

The US is not just a member of NATO, they are a member that surpass the remaining members (taken collectively) in every regard. Its not just the equipment or manpower lost but the ability to launch a military intervention around most of the globe, something the US, and only the US can do with combined arms. We also lose most of our effective and experienced leadership, technical capabilities like spy satellites and GPS.

If the US want to invade Europe, they can do so while mostly being sure that we (EU) can’t effectively shoot back, with or without the bases on our soil, like it’s not even close.