They really aren't. The number of apps requiring Play Integrity grows every day, my own bank's app hasn't worked in years and I've long given up on it, I just use it on a second stock device now.
And Google has an answer to the "just install the APK from somewhere else" workaround, too. Many apps now integrate a check that prevents them from running if they're not properly linked to the Play Store.
Are banking apps much more useful than banking websites, anyway?
Depends highly on the bank and what part of the world you're in. Some banks have only a website and no app. Some banks have only an app and no website. Some require an app to access the website. The landscape is widely varied.
Don't know what the landscape is like in the US, but here in Europe, many banks require the app for any sort of online payment. There is no alternative. If your phone isn't stock, you're screwed.