that's not entirely true.. any serious photographer or artist starts out with dark pictures just so that when they increase the brightness, all the existing luminousity is preserved. If you start too bright, a bunch of the colors are washed out if you go dark from that point on.
It doesn't work that way with computer rendering and gamma though,only photos and other static images, so there is no reason these screenshots couldn't be bright by default as they come directly from game engine.