This doesn’t have anything to do with flat design. It’s the fact that people take hammers and look at everything as a nail and go pounding things. Everything like this is a “contagion”, people just latch onto hot button ideas and go crazy. Flat design in itself is fine, and extremely beneficial for what it was designed for, it’s just overused because people chase trends.
Before flat it was skeumorphism and that was even worse. You had everything in tech trying to look like real things which made things way too busy and hard to read. And then people tried to make it work on tiny phones with low res displays and it was difficult to use.
Hence, flat design was born as a solution. It made icons easy to read on tiny devices. And it did a good job at that. It solved a problem and did it well and everything was well and good.
The problem was the next step where people decided they needed “consistent branding” so they did it on their website too. And then their marketing materials. And then their products. Then you had a problem.
Flat design works well for what it was made for: iconography. And for legibility of small UI. But it’s not for everything. But people can’t think for themselves and solve different problems in different ways. And Google made it easily available everywhere. And people picked that up and use it everywhere. And THAT is the problem.