To be honest, because they're arrogant. It's the same reason they don't adhere to each OS's UX/UI guidelines (including on Windows!) It's an absolute arrogance that they are "above" these platforms.
Well microsofts UI style changes every ~3 years, and they don't restyle old stuff, so at any point in time there are 10 or so possible correct stylings...
Also, it's the same arrogance that makes them push AMP so much. What AMP does is awesome just not how they do it. The wrapper, the URL hijacking, the janky scrolling, no reader view ...etc. It completely breaks my normal flow on my phone that I end up making DDG the primary search provider and Apple News (which is not good at all) as the news aggregator.
it's not just arrogant, it's fraud:
https://legal-dictionary.thefreedictionary.com/fraud
"A false representation of a matter of fact—whether by words or by conduct, by false or misleading allegations, or by concealment of what should have been disclosed—that deceives and is intended to deceive another so that the individual will act upon it to her or his legal injury."
Asking for admin rights on install - ostensibly for installation purposes only - and then by concealment retaining those rights for other undisclosed purposes without further gaining the consent of the user seems like fraud to me.
But it's also correct to say that this is how the median software vendor typically does business.
I'm a long way from the valley and regard googles behaviour as arrogant in the extreme, they are 1990's era microsoft with better PR, I don't trust them at all and that they seem to have taken control of the web via soft means is worrying as hell.
This is what happens when they systematically hire fresh grads that more keen on solving interview quiz puzzles than actually building good software.
This kind of a mess shows a clear lack of seniority leadership.
To be fair, not even Microsoft does that.