These are not strictly a result of targeting browsers for desktop applications, it's strictly down to design decisions that have nothing to do with how its implemented. I've seen plenty of wonky desktop/native applications that took way too many liberties with design whimsy that end up being a worse user experience than any webpage.
I also can't complain about Slack being slow because it doesn't seem slow to me and I'm part of a very large organization. I also use VS Code which is based on browser tech, and it's working really well. And I love that VS Code also works in the web applications I create. YMMV.
It's not "strictly" down to this; even if you want to implement interfaces that fit in with native ones, web browsers simply don't expose many native features via the DOM/CSS. It's a question of actual capabilities, not of this hypothetical design process that doesn't care about native integration.
(Of course, there will always be terrible native interfaces. Arguably Apple is the worst offender here!)
In an application like VS Code, or anything built on Electron or other similar frameworks, the web browser actually can have access to anything and everything a desktop application has. Electron is a fusion of a Chromium web browser with Nodejs, so you can call Nodejs functions from the browser, and if you really need OS API access, you can also have C++ addons for Nodejs that can do the work and the browser interface can be used to call those functions.
The web browser is simply the user interface, it communicates with a nodejs back-end which can definitely also access OS level APIs if you really want it to.
So yes, the capabilities are there, you just didn't know about them because you think it's simply a web browser, when it's much more than that.
And I know for a fact that you're wrong, because I worked on a desktop application in 2006 that used Internet Explorer as the user interface (Windows was our target, for reasons), and it had a C++ "back end", which was used for burning DVDs (the first legal DVD burning application). The browser could call C++ functions that we chose to expose to the browser to do all sorts of OS-level things. There was nothing that the browser front-end couldn't access at the OS level if we chose to expose it to the front-end. So I know for a fact that you're wrong about your assumptions about desktop applications that use a web browser as the front end.
Have you tried using a different theme?
Compared to what? I honestly don't know what other people consider state of the art interfaces if native apps are excluded.
