My (admittedly spontaneous) thought was that we don't see them anymore because flat design make them almost impossible. Even with dividers, it would just look like a grid.

I don't think that more-tabs-that-can-fit-on-one-"line" is ideal, but I think the modern equivalent - horizontal scolling tabs - is waaay friggin' worse. I've been confused by those before whereas the first time I encountered multi-row tabs in the "depth style," I had zero confusion about what I was looking at.