Maybe not for you, but rooted phones are a legitimate risk for users that do sideload pirated games and malware etc. I still think the risks are arbitrary, but I can understand why banks want to avoid rooted phones
Then they should also avoid rooted Windows machines where malware is a legitimate risk and orders of magnitude more prevalent than on mobile. Doing one but not the other is arbitrary and just pushes people towards having a locked-down device that they don't fully control with them all the time (can't uninstall tracking software from the vendor, for example). A locked-down computer that just sits in your office all day would be less worrisome