It's that they all look similar and you like the look. You also might be looking at websites built more by hackers than by web designers. Bootstrap is great for visually clean rapid development, but that comes at the cost of individuality.

Props to Coinbase for changing the looming black top bar anyway.

It's a theme from http://bootswatch.com/. I wish more sites used one of those, completely agree with you on the black top bar.