Because it's a waste of very expensive time to develop native desktop apps in 3 different platforms?
Facebook gave up on the one app and does native apps for the major phones for a reason: native is faster, ultimately more powerful and you have more control to fix bugs.
...But it's still better than Google telling the parent they have to install Chrome to install Google Chat.
The majority of the underlying libraries wouldn't necessarily need to be changed; "only" the UI layer and some of the APIs (which is non-trivial, but doable).
and yet the latest-and-greatest cross platform framework is supposed to be Flutter
In case anyone was wondering why Apple is going with the "iOS/iPadOS apps on the desktop" route, it's because this is so incredibly common; great, first-class mobile apps and nearly unusable, garbage Electron-based desktop apps.
Edit: Plus, in regards to Google, something about Chromebooks. A good desktop app on a Chromebook is just a web app that works well in Chrome. Why make a desktop app for what they seem to think is just an outdated computing metaphor using APIs they don't control?
My aunt is pretty locked to Google platforms, she is the person who trains their platforms at her work... and you know what, the idea that she could "just use Skype" didn't even strike her mind:
It's already installed on my grandma's Windows PC, she already has a Microsoft account anyways, and it "just works", popping up a "[aunt] is calling] dialog when she gets a call. Skype worked great, and it's available on every platform.
It really starts to amaze me that Google doesn't understand the importance of that level of integration and simplicity considering their supposed market being "everyone". My aunt had been considering buying a Google Home/Nest/whatever it is today or an Android tablet, just to use one app to call my grandmother.
But my grandmother already has her PC open, and Skype "just works". It rings like a phone and it has a big green button to answer it.
A few days ago I tried to install Google Chat, for work. There's no download buttons to be found, you're just logged into Chat directly and is greeted by a "You need Chrome to download Chat". Well that just weird, but okay, I can uninstall Chrome afterwards, no problem. Imagine my surprise when launching Chat also launched Chrome. I disregard this as being something Chat just do on first launch. Existing Chrome however also closed Chat.... Okay. Uninstall Chrome then, that should teach Chat to no fool around with Chrome. Now Chat doesn't work, because what Google doesn't tell you is that Chat is now a progressive web app. The icon is pretty though.
Chat can be a web app all it want, but it doesn't get to launch Chrome, I don't want that abomination running all day. The current fix is apparently to install via Home Brew, that downloads a stand-alone app. Google claims the new version is stand-alone, but if it was to launch Chrome it's not really stand-alone is it.
What struck me about all this is that Google doesn't actually have the ability to develop real desktop application. That's really weird, why wouldn't they want to have the best performing, most powerful applications possible on the desktops? Because they can't. Google as an organisation doesn't have the mental capacity and patience to do real desktop programs.