Banking apps sometimes integrate 3rd party AV / threat detection SDKs (there are several available, e.g. https://www.wultra.com/malwarelytics which is part of a bigger banking security SDK solution). Sandboxing isn't the only problem on Android - a lot of malware is using Accessibility to control the device and read what's on your screen. Then there are overlays that can be drawn over your app (e.g. display a fake login over your app). Access to notification content. Reading SMS content (2FA). Different combinations of other permissions, device manager rights etc. Or even simple attacks like launching a fake app just when you launch the real one (phishing). Google is trying to limit a lot of these permissions, at least from apps installed from Google Play but so far they are still pretty widespread with malware hidden on Google Play targeting banking apps.