Many many people have banking apps that will not work on non-Google-blessed devices, use banks that have mobile websites that are terrible, and need to do mobile check deposits (which is usually only available in the app, and not the mobile website, if the bank even has one). And no, we're not going to "change our bank".
The reality is that there are so many things that break, sometimes in subtle ways, when you try to use an alternative Android OS. Some people may not have any problems, and that's great! But many -- I would dare to say most -- will.
And there's also a ton of uncertainty: I don't really want to wipe my phone, install GrapheneOS, spend hours messing with it and setting it up, only to find that something critical doesn't work, and now I have to flash back to the stock OS, and hope I can restore everything the way it was.
I do see five banking apps I use listed there as working, which is great. But -- and maybe I'm being unnecessarily overly worried about this -- what about the future? What if I've been using Graphene for a year or two, and one of the ones that's critical for me changes how they operate, and Graphene no longer passes muster as a platform it will run on. I'm not afraid of this happening at all running Google's stock OS image, but once I do my own thing, I get to keep the pieces when it breaks.
GrapheneOS is the way that all phone operating systems SHOULD be made. Layers and segregation between your banking apps and all the privacy breaking trash and malware you can get off the app store.
It is the banks and google making weird rootkit shit to try and lock down things that is the problem here.
There is also an alternative for now, but nothing as simple as SMS or authenticator app. They give you a special credit card shaped card with a card reader that you can use to authenticate with using your PIN, which is mostly considered legacy now with the bank app. It's also not realistic to be carrying this thing around everywhere either as it's bigger than my phone.
There is also a national ID app that is used everywhere that I'm worried will stop working on GrapheneOS... Because without it I won't even be able to access online government services like healthcare, taxes, etc.
They have a nice web app, but you must use their mobile app to login on the web version. The app takes a video of a QR code on the web page during login. Web login completes as soon as the mobile app notifies the server. There's no 2FA code to enter, and no alternative.
I asked them about this, by phone call, when my phone screen broke and I urgently needed to make a transaction. Surely there as an alternative? Or could I do the transaction by phone call?
They told me that indeed there is no other option. Despite having phone customer support, they had no phone or web banking service at all which could be used without a registered mobile device. The only phone service they could perform was to register a new mobile device, which I didn't have. I had a tablet, but it was too old.
So I had no good choice. The Android phone I'm using right now was bought in a hurry just so I could be allowed to make a bank transaction.
It wasn't my first choice of phone. I didn't have time to investigate alternative devices, let alone weigh up open alternatives. I ended up buying a mid-range device under pressure that seemed ok and was available in a store without waiting. (It was a brand new Samsung, and despite the IP rating it got water damaged and stopped working entirely after a few splashes a year or so later, but I was able to get it repaired.)
That's not any OS' fault, that's banks fault. That's been my experience with every bank I've used so far and yes - they often break on certified OS' too! I've been on the phone with support!
Because they make bad software, period, and we're all just forced to use their bad software.