Why don’t Europeans provide troops for Europe’s defense?

Three years ago the U.S. withdrew combat units from Europe. Now it is sending them back to deter a Russian attack. Explained Brig. Gen. Timothy Daugherty: “Preparing for war is a lot cheaper than executing one.”
True. But why isn’t Europe doing the preparing?
During the Cold War the U.S. had about 300,000 troops in Europe. The number was down to 65,000 a couple years ago. Even so, that was too many: the continent long ago should have graduated from America’s defense dole.