[alpaca128]

> I guess the behavior of tabs within splits is baked into most GUI APIs and frameworks

Yes, that is probably a common assumption. On the other hand it may not be that hard to just do the tab bar and split containers separately and handwire them together via input events. At least web UIs aren't exactly famous for their adherence to any standard.

So the more likely reason is that most developers just don't have a reason to do it. Because in the end Vim's approach is just more an exception and as far as I know Vim tabs weren't even originally intended for the same purpose - and I can confirm buffer switching within a tab can be pleasingly fast and precise thanks to the tab completion.