Isn't it because we're stuck in NATO?
Without the US, NATO is a paper tiger. We supply most of the firepower, logistics, and money. Only 8 members meet their commitment of at least 2% of GDP spent on defense. NATO membership represents a life insurance policy that provides a security guarantee that the U S will act as the guarantor of their national sovereignty up to and including the use of nuclear weapons. An attack against one is an attack against all.
The US has provided Europe’s security umbrella since the end of WWII. While the US taxpayer pays most of the bill, Europe has constructed a generous social welfare system without the need to spend significant amounts of money on defense. Uncle Sap has been paying most of the premiums on this life insurance policy called NATO. Membership has been a great deal for the European members. For the US, not so much. NATO expansion has been a disaster increasing the risks for the US with no real benefits.