There is always some lag when it comes to being safe to use certain features, while it's true that flexbox is quite old, consider that at the time it was released many people still had to support browsers like IE6/7.
There was also a problem that the spec changed so at one point you had to support two versions, which put a lot of people off adopting it for a long time. The point is still that this stuff isn't new though. Browsers don't change very quickly, especially for the fundamental building blocks of how you get a website on a screen.