For many years now KDE has focused on polish, bug fixing and "nice-to-have" improvements rather than major redesigns, and it paid off.
I often wondered why desktop UIs became so terrible somewhere in the 2010s and I don't want to attribute it to laziness, greed, etc... People have been lazy and greedy since people existed, there must have been something else. And I think that mobile is the answer.
UI designers are facing a really hard problem, if not impossible. Most apps nowadays have desktop and mobile variants, and you want some consistency, as you don't want users to relearn everything when switching variants. But mobile platforms, with their small touchscreens are completely different from desktop platforms with their large screens, keyboards and mice. So what do you do?
In addition to mobile, you often need to target the browser too, so: native desktop, native mobile, browser desktop, browser mobile. And then you add commercial consideration like cost, brand identity, and the idea that if you didn't change the UI, you didn't change anything. Commercial considerations have always been a thing, but the multiplication of platforms made it worse, prompting for the idea of running everything in a browser, and having the desktop inferface just being the mobile interface with extra stuff.
Who do you think has been "infected" by the "mobile" virus? KDE's only real competitor is way more keyboard focused than KDE...
[1] Hamburger menus are designed to make efficient use of a small vertical display where horizontal screen space is a limited commodity, which just is not the case at all for a large horizontal computer monitor. On a large horizontal display, they're a straight downgrade since you need to click the menu to see what's inside it, which makes action discovery harder. This click is also added to a lot of actions so they add more friction to almost all interactions.