Hackery*, and I'm not sure why the hate towards CSS3. With a few lines of code I can build a pretty good looking responsive UI that doesn't take hours to polish.
Desktop centric UI toolkits provide a set of widgets which lets me build up a UI in a predictable way that respects my sanity.
Windows Forms, WPF, Qt, vxWidgets, tkinter, Java AWT, SWT etc they all have warts but ALL of them feel faster than building UI for the web.
The problem with web is that there are no consistent set of widgets.
Let's take a stupid date picker. A year ago for a project I thought I'd use the HTML5 <input type="date"/> way no Javascript needed!
Sadly I couldn't figure out a way to keep the calendar in an always on state so had to resort to jQuery datepicker. So jQuery widgets was an attempt to do what desktop UI does but now that is no longer in vogue.
Today on the web there is no consistency, there is no agreed upon toolkit. How many different approaches to widgets does React have?
Ideally you would be able to import a web component into your project and use it without worrying about any abstractions leaking.
Moreover, it's unfair to insist on portability when comparing native vs. Web without doing the same on the native side.
There plenty of deployment scenarios with other kinds of browsers.
Of course it is fair, using something like Qt, wxWidgets, or even roll our own was never as complicated to get pixel perfect UI across OSes.