The only thing that defines portability is everyone adhering to the same standards.

You say that the web is portable, but really, only Google's vision for the web is relevant, seeing how they have the final say in how the standards are implemented and evolved.

So it's basically another walled garden, only much bigger and not constrained to the CPU architecture and OS kernel.

Chromium IS a platform. And indeed many applications that do work on Chrome don't work on Firefox. So we're pretty much back where we started, but the problem is harder to see because Chrome has such a monopoly over browsers that for most intents and purposes, and for most devs, it's the only platform that exists.

Everyone is good at multiplat when there's only one plat.

QT has been around for decades. So has GTK. Bindings for whatever language you could possibly want. Runs on whatever OS you want. We've had "truly portable" UI frameworks since the late 90s. This has not been an issue for my entire adult life. 20 years ago, I was using desktop applications that ran on Mac OS X, Windows, and *nix with no modifications. They were written in Python, used GTK, and just worked.

Web apps are popular because 1) people don't like installing things anymore for some reason and 2) it's easier to justify a subscription pricing model.