From what I understand other countries (especially EU) essentially contract out defense to the US for economical reasons (i.e. so they can afford to spend more of their budget on social services and less on military). If US cuts military budget it means EU will have to increase military budget by
more than the US cut to make up for it.
The biggest EU defence needs are two: securing oil ingress from the Middle East and securing resource ingress from Africa. Without the US 5th fleet, Iran or any other regional power could shut down oil ingress, pitching the EU into a sizable (-30%) GDP recession. Instability, of which there is plenty in the Middle East, could do the same.
Same for Africa, with the added threat of Chinese competition for the same resources. Without those raw materials (minerals, uranium, some oil and gas) European manufacturing doesn't have a future. Hence the constant French and American interventions to stabilize various African states and their tight relations with their governments.
That state of affairs is because of the Post-WWII order the US constructed via alliances & multilateral organizations, in response to being repeatedly dragged into European wars.
The only reason they are not in need of defence is because the US umbrella still protects them from invasion by foreign powers. If the US were totally uninterested in Europe, Russia would likely make much bolder moves in the region.
If they feel they're making the US pay for their defense spending good on them. On the US side of things a good chunk of the country thinks we should be world police for patriotic reasons - another chunk of the country thinks we have a duty to defend the world... While the majority of people (who are underrepresented by special interest groups) think it's insane we get five F35s for a billion dollars.
