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.
To be fair, not even Microsoft does that.
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.
Where are the industry veterans at Goole?