I am so curious about it and can’t find an answer. Apple, Facebook, Twitter, Google, etc.
Can you imagine flat design from Apple? Probably never. By flat I mean using ANY of the features of this approach (not merely copying the whole thing). For instance, ALL those companies use very small elements (buttons, fonts, icons) , with much higher density.
So why basically they all choose not to adopt it? Is it because it doesn’t speak to the mass? Seems there is a single reason shared among designers.