If your bank really spies on you through its app, I would change bank. Neither of my bank apps even run in the background or even request sensitive permissions. I will happily change my mind if you can show any proof that this is happening.
It's purely security. On Windows and largely on Linux desktop as well, any app can easily look at other app's data, that's why there's so many browser credential stealers. Maybe you'll never be a victim of this sort of attack, but if it does happen your bank account is gone.
Android and iOS have complete data isolation between apps. Unless you have root on it, even if you install malware and give it the maximum amount of permissions Android can possibly give, it can't access your auth cookies from the bank app. The bank app can't even access them either until you input a pin or biometric data to get it from the TEE.
Thus it's safe for banks to actually let people stay logged in with reduced identification. Browsers can't do that, not without the web integrity.
We're an absolutely minuscule minority that cares, and could use a stay logged in feature safely in a browser environment.
Dealing with fraud cases is expensive for the banks, they have good reasons to ensure you can only access your bank account under safe conditions. The average person doesn't even know what a web browser is, they know they click the Google and enter what site they want to go to into Google and search for it. They're the people that get scammed on the phone. They're the people that have their entire life savings wired overseas.
Just let your password manager fill up the login everytime, it's not hard.