1. These companies themselves don't practice what they preach. No matter how fast Speedometer 3 is, Google's own web.dev takes three seconds to load a list of articles, and breaks client-side navigation. Google's own Lighthouse screams at you for embedding Youtube and suggests third-party alternatives [1]
2. The DOM is a horrendously bad, no-good, insanely slow system for anything dynamic. And apps are dynamic.
There's only so much you can optimise in it, or hack around it, until you run into its limitations. The mere fact that a ToDo app with a measely 6000 nodes is called a complex app in these tests is telling.
And the authors of these tests don't even understand the problem. Here's Edge team: "the complexity of the DOM and CSS rules is an important driver of end-user perceived latency. Inefficient patterns, often encouraged by popular frameworks, have exacerbated the problem, creating new performance cliffs within modern web applications".
The popular frameworks go to extreme lengths to not touch the DOM more than it is necessary. The reason the DOM and CSS end up being complicated is precisely because apps are complex, and the DOM is ill-equipped to deal with that.
This only goes to further show that browser developers have very little understanding of actual web development. And this is on top of the existing problem that web developers have very little understanding of how fast modern machines are and how inefficient web tech is.
This brings us neatly to point number 3:
3. Much of the complexity on the web in the modern web apps is due to the fact that the web has next to no building blocks suitable for anything complex.
https://open-ui.org was started 3(4?) years ago by devs from Microsoft and you can see just from the sheer number of elements and controls just how lacking the web is.
So what do you do when you need a proper stylable control for you app? Oh, you "use inefficient patterns by modern frameworks" because there's literally no other way.
And even if all of those controls do end up being implemented in browsers, it will still not be enough because all the other things will still be unavailable: from DOM efficiency to ability to do proper animations to ability to override control rendering to...
Why wouldn’t this?
Bad developers (or management dictates) will be bad no matter what. That’s not a reason to give up.