GUI's used to be flat. Windows up to 3.1. AmigaOS up to 2.0. MacOS. TOS. Athena widget set etc.

They were all flat, and to a certain extent colourless.

We added bevels and colour because we could, and because it stood out against the competition, and it was fresh and new.

And then we piled on the bling next time around because the entire competition had beveled, colourful UI's.

And it carried over onto the web, because we could and were already using it everywhere else.

And now it's flat that is fresh and new again.

This is trends and fashion much more than it has anything to do with familiarity and usability.