I'm not sure the web is embracing flat, it's more like designers get bored and feel the urge to do something. They have to justify their paycheck and raison d'etre.
It's like the fashion industry, what is out of fashion comes back in because somebody says so and the catwalk is then flooded with retro fashion.
People use this tired old argument on us too - "what? Why is IT building the website again? Don't we already have one? Welp, guess they have to justify their paychecks somehow...", it's tiresome, reductionist, grossly ignorant, and textbook HN: everyone is useless except me.
Fashion isn't a simple cause and effect - it isn't designers dictating taste to an unwilling public. The influence is circular - design influences public aesthetic influences more design. The fashion industry steals plenty of ideas right off the streets, it's far, far from a one-way relationship. "Fashion" is just an endless cycle of the public influencing products and vice versa. Sometimes it will retread familiar territory, other times it'll do something new.
And thank god we have it. I for one am glad we're rid of animated GIFs and fluorescent green text, or do you think that evolution is also bored designers justifying their paychecks?
But nah, none of the above can be true, everyone's a useless parasite except the vaunted Hackers, hallowed be their names!
It's like the fashion industry, what is out of fashion comes back in because somebody says so and the catwalk is then flooded with retro fashion.