It's more than that—it's peacock feathers. You do it to signal you have money to burn looking "modern". You'll even see people on HN complain that a site looks "old", sometimes—not bad, just old. Keeping up with trends signals you're doing well enough to keep your site from appearing old, so looking maintained and up-to-date.
I think it's related to how chain stores & restaurants look so much more polished these days, and completely re-design their logo and colors (repainting whole buildings) every few years. They didn't used to do that, and everything generally looked a lot less "slick" than it does now. All I can figure is a couple of them started doing it to differentiate themselves from smaller chains and mom-n-pop stores (that couldn't afford that crap) and the rest started doing it so they didn't look left behind. Such a waste.