Literally billions of regular users cannot do that, and regularly install malware laden apks.
I have no way to easily determine that an app on the Play Store actually came from my bank. At best, I can look at the install count, hope the app store does a half way decent job looking for fraud, and hope for the best. Maybe if I'm lucky they have some sort of verification program?
The idea we actually trust app stores from Google or Apple over a direct install from the institutions we want to interact with is hilarious, especially considering all of the rampant malware and fraud issues on the Play Store.
If they dist an apk, how is the bank to provide updates for their app? Build their own updater architecture? If there was a mitmed update from a store they could cancel it centrally.
Telling people to prefer apks is setting them up to be hacked.