And the real kicker is that most of these aren't done well.
Aside from having a very limited expressive range, flat design has all kinds of usability problems that plenty of people have called out from the start. The fact that both Microsoft and Apple have chosen to adopt flat design does not change that unfortunate reality.
I cringe a little every time I see a web site using Proxima Nova, a font which renders horribly at many sizes and in many browsers, whatever all the renting-you-fonts-for-money services will tell you about how optimised their fonts are. I wince even more when I have to use a site with trendy thin fonts that are so hard to read under many conditions that I have to start hacking the CSS around to make it more comfortable. At least there is a small silver lining here: Helvetica is less common in font stacks now, so everyone on Windows who has printer fonts installed can breathe a sigh of relief.
Carousels have problems that are well known to anyone who studies usability, yet people persist in using them, presumably because they care more about looking cool in a demo than having a site that is actually effective. Or they just don't know any better, because they don't know anything about usability, and anyway they feel compelled to use at least 98% of what's in their template on every project. After all, the only acceptable alternative is to use a hero image that automatically scales to full page width, thus reducing modern, good quality, high resolution monitors to displaying heavily pixellated versions of clip art that is probably irrelevant anyway, and obviously no designer would do something silly like that.
It's not the consolidation and uniformity that bothers me. It's consistently doing things that just aren't very good that I find frustrating.