Probably because your desktop apps are bloated, too. Any software that needs longer than zero point two seconds reacting to user action (this includes starting the program) should be improved upon.
I write moderately complex web apps and I have to say 200ms for a user interaction sounds like far too long. All apps regardless of where they're running should be managing a solid 60fps, which gives you a budget of 16ms for the UI to react to a user event.
Obviously starting the app is going to take longer, especially from cold, but if you ignore things like DNS resolution and download time then 200ms is more than enough for most things to get from 'downloaded' to 'interactive'.
It shouldn't be obvious that startup is slower and I didn't mean UI reaction, but the more broader reaction time. For example from typing a location and hitting enter, it should take something like Google Maps zero point two seconds at most to completely load the page and be done with everything. The user shouldn't have a concept of "loading time" and that includes starting the application.
I think the only native apps that I use are Steam, Unity, Photoshop, SourceTree and VSCode. Out of those the only "fast" one is VSCode, which is actually written in JS. Even though you mentioned the browsers being "bloated", for me Chrome always starts up instantly and I have no responsivness issues when browsing sites, playing browser games or using browser apps.
Out of those the only "fast" one is VSCode, which is actually written in JS
Typescript I think? Anyway, much as Microsoft has done an amazing job with VS Code, it really doesn't have quite the snappy feel in all circumstances of a native app. Extension loading on start up is one example. If you use a keyboard mapping extension it's quite possible to start typing on launch and find the mapping hasn't kicked in yet.
That's not to say it isn't worth the tradeoff. The costs are inconsequential, and the benefits great. I'm mostly an IntelliJ Idea user, but the sheer speed with which VS Code is developed often tempts me towards it (eg. it's well ahead of Idea on wsl integration)
I do like performant apps, but I've never understood the emphasis on startup time for most of them. There's the odd utility (eg. screenshotting) where an instant start is desirable. But the main things I use (terminal, editor/ide, browser etc) are started once per week at most.
I personally wouldn't want the developers of the software I depend on most to waste resources speeding up something of such marginal significance.
[Edit: an instant-start editor for config file, vim commit edits etc is of course crucial]
I have a workflow where I spawn and close terminals depending on whether I want something done. If my terminal startup wasn't instantaneous, I'd be switching to one that was. (Currently using urxvt). The only reason I keep my browser open is because it doesn't start instantaneously. Imagine a world where your computer and all your apps can start instantaneously. It would be viable to shutdown your computer when leaving the room instead of just locking it.
In this reality, however, I would already be happy if tabs in a browser could load that fast, so I wouldn't have to keep hundreds of them open all the time.
Obviously starting the app is going to take longer, especially from cold, but if you ignore things like DNS resolution and download time then 200ms is more than enough for most things to get from 'downloaded' to 'interactive'.