Answer :
Europe is the origin of modern civilization and intelligence. its cities are very old and have had much more time to grow in population than the united states.
Europe's cities are so crowded because the population is overgrowing and more and more humans are coming to Europe. Moreover, the number of people crammed in has overtaken those in Holland, long the most densely-populated major nation on the continent.
Hope this helps! :)
Hope this helps! :)