Because I'm the owner of the machine and I decide what informarion the applications running on it can obtain. If I want them to believe they're running on an unmodified operating system, then that's what they should believe.
> it could have been tampered with by a malicious actor who can now lift your bank keys from the device
Then give me the keys to the machine and let me use them to sign my own software. People who don't care can use the easy corporation attestation but I still get to maintain control.
> You modified your OS to do something you couldn't do on an OS that wasn't tampered with.
> If instead, the thing you wanted to do was available behind an appropriate permission, you wouldn't have had to modify your OS to begin with.
And what if what I want to do goes against some corporation's terms of service or whatever? What if I want to monitor what apps send over the network? Reverse engineer an app? Copy and manipulate their data? Automate tasks? Fake the personal information returned by system calls? Block advertising? Block surveillance?
Essentially, what if I want to do anything that harms their bottom line? These are not things that corporations will ever give me permission to do. It's an affront to my freedom as a computer user.
You can do all of that on Android, many of those without root. If you want to modify any process's memory, you need a permission that doesn't exist. With sufficient imagination, you could invent a permission system that allows you to do that but doesn't allow a malicious app to do that. If instead you allow arbitrary tampering like you want, up to and including replacing the system partition without indication, there's no way for the bank to know your banking has been hijacked by somebody other than yourself. Your bank doesn't give a damn what you do with the banking app on your device. It cares if it has to deal with mass exploitation of its customers.
And why not? If it's tampered with, it could have been tampered with by a malicious actor who can now lift your bank keys from the device.
Ultimately, it is about permissions. You modified your OS to do something you couldn't do on an OS that wasn't tampered with. If instead, the thing you wanted to do was available behind an appropriate permission, you wouldn't have had to modify your OS to begin with.