Yeah, America in general is FAR more contemporary than Europe. For one, we don't have as much history and tradition behind us as you guys. Only a couple centuries, while London's been around a couple millenia.
I think the reason DC shares similarities is because it was burned to the ground. Cities that have been burned to the ground seem to have a lot of history and monuments. Okay, I'm kidding,bBut yeah, DC is kinda the icon of American patriotism. While everything else is focused on capitalism and commerce, like, say, NYC, DC has been all about law, justice, country, etc. So its a logical place to iconize our heritage, which is probably why it has more monuments than any other city.