Thats part of it, the other part is simple market economics. It is hard for the things that people want to do at night (clubs, art galleries, restaurants and bars) which are much lower margin businesses to compete with the large financial and governmental institutions that dominate places like the Chicago loop, lower Manhattan or downtown Dallas.

These leads to a pretty common pattern where just outside of the financial/governmental centers of US cities there exist neighborhoods that are mixes of residential, cafe's, nightlife, "creative businesses" or retail. For instance, the Chicago loop is literally surrounded by such places.

But the lack of activity in a financial district has very little correlation to the density of the city. Manhattan is obviously a walkable and dense place to live but lower manhattan (wall street) can feel down right empty after hours.

I have much less experience in European cities, but my experience in Dublin, Amsterdam and Frankfort are not much different than this.