Phones are generally seen as more secure because they're less likely to have malware and the apps should be running in their own sandbox, meaning it's more difficult to see what each app is doing and so theoretically it's more secure.

Most desktop operating systems do not have sandboxing in place, have known malware that could be installed much easier than on a phone, and harder to verify that the system is secure. This is doubly so taking into account that basically the only way to use the banking information is through a web browser, which could have any number of junky web extensions installed.

While things are incrementally changing on the desktop front (mostly on Linux with Atomic distros, Flatpak/Snap, and Firefox container tabs), most banks are only familiar with Windows and macos, and since those two have the most security risks, they'd rather play it safe with the relatively more standardized, theoretically more secure phone OS.

They're more locked-down by the manufacturer than desktops, so they're more "trusted" by corporations to act in corporate interests at the expense of yours.

As things stand, mobile OSes have some pretty decent out-of-box ways for apps to be isolated. App A can't fiddle with data private to app B. That Android video game you just downloaded can't extract data from your web browser or generally fiddle with it.

Desktop OSes today don't normally have software install and work like that. Yeah, you can manually set something like that up with containers or VMs, but your typical user isn't going to do that.

Not really, banks are simultaneously really smart and really stupid about security. They do incredibly annoying things that don't do anything or are negligible security wise all the time

Some bank apps won't work if they detect your phone is rooted for "security" when root just gives you the ability to grant administrative access to apps. And yet this is the default way desktops/laptops operate.

Some banks refuse to let you pick your own username and instead assigns you a number that's sometimes random and sometimes just your primary account number. Why? "Security" and just for even more "security" you have to wait for them to send you that info and a pin through snail mail

I trust my mobile device much less than my desktop.

I'm with you. Phones are toys, a PC with disk encryption and well chosen software it way more trustworthy.