You may love the way it looks... but no one being honest with themselves loves the way it functions, UI is visually functional, a functionality that happens in your brain, like legibility - flat UI has very poor "legibility" if compared to something like 3d embossed win95 UI where every identification and anticipation is painfully obvious to the point that it's effortless. It's familiarity is also learned in a very short space of time; whereas flat UI continues to elude even advanced users and remains ambiguous with high cognitive load - you might not realise why, but it will feel slow, and uncomfortable for each new front end you encounter.

Designers don't like this fact because that embossed 3d look is considered ugly, but it's intuitively and obviously true.