There's an entire world outside USA and EU and for EU it's way cheaper (and simpler) to have the US dealing with that - an approach having its plusses and minuses of course, like any compromise.
It's just self-interest on both sides, Europe doesn't have to be grateful any more than the US has to be grateful to the Europeans for allowing bases on their territory.
They should just stop doing that if it's such a trouble.
Of course, we know they won't, because it serves US purposes. You are just too dense to either see or admit that. Keep pretending that the US is some sort of self-sacrificing entity, just to protect some ungrateful Europeans Across the pond.
We are also pretty much at the point of missing military recruitment numbers by 20-25% every year now.