I agree, although there are now more platforms to deal with than back in the days when it was just Mac and Windows: add iOS, Android, desktop web, mobile web.

I think it also helps when it's easy to share code between platforms, which again, used to be easier when you were just targeting the desktop (just use C or C++).

Now it's a bit harder because there are no great languages that span all platforms easily without a bit of hacking. Most languages are managed and memory-safe now; that's a good thing, but it makes them more heavyweight and less portable. They generally want to drag in their own frameworks (e.g. Flutter, React Native) rather than letting you put your own UI layer on top.

This problem is already solved by virtualized lists available in any decent framework. Why people don't use it is beyond me.

I think in the case of something like Facebook, the sheer complexity of their product drives a need for a singular code base. It would slow their engineering organization to a halt to ensure feature parity with largely seperate code bases.