It’s not purely a question of spending. There are qualitative differences as well. No US ally has even an approximation of the US nuclear deterrent or the US Navy. Partially as a consequence of this, no US ally can match the quantity, quality, or versatility of American air power.

It could be, that's part of why Trump flirted with the idea of making NATO's 2% of GDP military spending target a requirement rather than a general guideline.

However, the fact that the US is the main component of NATO gives the US a lot of leverage over those other countries which is valuable in its own right.