In all the most read languages, text is read most easily horizontally. That means that if you want to be able to read the tab titles, they need to be very wide. If they are stacked on top of each other, they can have a fixed width that you're willing to sacrifice, and then you can read the titles easily and scroll through them quickly. They pack very tight (one line) vertically. They don't compress as much horizontally while keeping the titles legible. Using only icons and packing them tight is hard to parse, because horizontal lists are harder to parse than vertical lists.

Further, because monitors are so wide, even one line (and especially one line with all the padding that is required for a UI element to be comfortable to parse) spanning the entire width of the monitor is a felt sacrifice. The width of a normal website title sacrificed horizontally for the entire height of the window on the other hand isn't felt as strongly.