If you run a rooted phone and download malware, that malware can gain root and do payments on your behalf. Then PayPal has to deal with you revoking payments etc., they don't want to so they forbid you from using PayPal on a rooted phone.

Malwares can possibly do that even on non rooted phones if a privilege escalation attack is possible. And just yesterday, there was an article here about exactly one of those.

Also I highly doubt that there is any real statistics anywhere about whether this is a real threat or not. I guarantee that nobody did such statistics properly. The only known data is from companies which sell root prevention tools, so totally unreliable. And internally I guarantee, that no banks collect such info.

So no, banks lie about this only because they can sell this to judges as safety feature, when they fuck up, which happens continuously.

Graphene isn't rooted.

If this happens it's the device owners fault and they should be responsible for it.

For that argument to hold, they'd also have to blacklist any phone not running the newest, most up to date Android version, because all older versions presumably have known exploits. So that basically leaves Pixel phones.