[HereBeBeasties]

Because on mobile the primary interaction point is your right (or left if you're a leftie) thumb, which naturally falls over a tab bar at the bottom of a screen. Whereas on the Web the primary click zone where your cursor focus is, is at the top of the browser window where the navigation elements are.

There's also the obvious stuff about top-to-bottom reading order making such a layout making the navigation much more obvious when you first look at a site (which is probably want you want for most things, although not all, which is reflected in things like blogs often putting ancillary navigation elements in the page footer).

I don't think this is a broken thing that needs fixing.

I'll also note that it has taken mobile UX designers YEARS to start moving navigation elements to the bottom of their apps, driven by larger and larger screens making elements at the top of the screen quite uncomfortable for one handed use. It's required that to become sufficiently cumbersome that the trade-off in immediate discoverability becomes worth it. And you'll note that the most popular mobile apps such as WhatsApp still have all their navigation elements at the top, regardless.