Having a menu that needs to be deployed makes the whole process slower -> I first have to open the menu, _and then_ I can start deciding which way I want to switch.
It doesn't seem like a big deal, but when you use the browser for hours a day it is, because there's this additional cognitive cycle of having to request information to be displayed first -- rather than have it always present.
Finally, the tabs on the bar on the top have virtually infinite surface and are super easy to click. You see, a 100x100px button has a defined surface where you have to point and click. The smaller the surface, the harder it is to click. But a button at the border of the screen has one dimension infinite: I don't have to aim accurately on the Y axis, I can move my mouse beyond the top of the screen and will still hit the tab.