There are two main reasons Mac has less apps than iOS.
1) Less marketshare
iOS marketshare is massive compared to Mac marketshare. Less marketshare the less incentive companies have to develop apps for the platform. I can't find any exact numbers on active Mac usage, but its probably somewhere around 5%-10% of iOS
2)Different API's were used to program the apps
Even though iOS and Mac use the same programming languages to make apps (ObjC or Swift) the API they have to use are different - UIKit for iOS, and AppKit for Mac.
UIKit is newer than AppKit, so it was easier to use to start with, and because of the huge sales iOS was making it was where Apple put most of their effort in improving things for developers. I have checked the AppKit stuff but honestly it looked like such a convoluted mess I gave up even trying to make a Mac app with it long ago.
Until recently* in order to support both iOS and Mac developers would have to have parts of their app in UIKit and parts in AppKit, this is a huge amount of work to undertake compared to just supporting one platform.
So we have the situation where most iOS devs did not start as Mac devs and most companies have to weigh the risk of spending a lot of time, money and effort to port to a platform where there is little benefit for them.
If we combine both the main reasons and apply it to a company like your bank we can see how it wouldn't make sense for them to make a Mac app.
1) Their developers probably only have a background in iOS, and maybe Web
2) They provide their app for free to users. So they can't make a good business case to directly put the time, effort, and money into a Mac app.
3) They already have a web site. This is good enough for most people who want to use their service on a Mac.
4) An iOS app makes sense because nearly all their potential customers have a smartphone, and web is still kinda **** on handheld devices.
* In recent years we have new options to bring iOS apps to Mac.
Either with Mac Catalyst to build and run iPad apps on Mac, or using SwiftUI - Apple's new cross platform framework.
Some caveats with them being:
- The Catalyst app must be an iPad app to enable this function. The number of iPad apps is much smaller than those made for iPhone. Also changes will still need to be made to make the app work best on Mac. And Catalyst supposedly doesn't feel as "native" as using Appkit.
- SwiftUI is very new and so some features are/were missing, and even though it's using the Swift language it is very different than using UIKit so many developers don't want to put in the effort to learn it yet.
Obviously the future of all Apple platforms is SwiftUI, Apple is putting most of its effort to improve SwiftUI as they did with UIKit in the past so places where it was lacking the year before are getting fixed and improved upon each year. As SwiftUI gains adoption we should see more iOS apps come to the Mac as the barrier to entry will be so low it won't make much sense not to make the app run on Mac