not only bank sotong, some other banks also.
i find it quite invasive that:
1) in order to use online banking, must use your buggy app
2) your buggy app force u to grant permission to your phone, location, storage in order for ut to work.
3) they mis-used thr permission grant to scan your ohone for other apps & might refuse to work if detected sideloading app.

not all sideloading apps are harmful.
there are many reasons to sideload legit app:
1) legacy app that is used on some of the older devices like older cam, older alarm system.
due to not mainstream, app is not updated by developer.
the existing apk is no longer officially supported on newer android, hence u sideload said app.
2) due to reason of bugs & compatibility, u prefer or have to use a specific version of that app.
u download that version apk & install it manually.
3) due to some app developer lazy, even though your os version supported, but your phone model not supported.
workaround is sideloading.
this happens to my boost wallet previously.
it will on & off, sometimes show supported, sometimes not supported in playstore.
so i resort to sideload.
4) i'm simply an app developer, or developing app for hobby & testing on my device.
ie, i can debeloped a highly specific app that automate 1 function that i use on my phone.
i dont intent do release for public, because this is for my specific use case only.
self develop, self sideload.

the fact that android allows sideloading shows that it's a function, & not wrong to do so.
but bank spy on my phone & treat it like i rob a bank or something.