tl;dr; - The US has government-owned (but contractor-operated) munitions plants, and boosting production is an actual (vs. lip-service) priority. The EU has a bunch of purely-private producers, all hog-tied with regulatory and financial constraints.

Cynical suggestion: An annual EU-wide "which country can boost production the most?" contest. Leading politicians from the losing countries are rewarded with six months in the front lines in Ukraine.