Every app captures its own user data that they sell. By forcing you to use their official app they are turning your data into revenue for themselves.

Webclients are trickier to maintain, because people use craploads of browsers and it might interfere/block/hack their online banking session.

Also, it's harder to steal your private data via browser, because it might prevent the attempt via ad/script blocker.

Take it as you want.

The main reason is to be on the users Home Screen, just one tap away from being on the app. Also, push notifications, pulling people back into the app.

Also there is a security benefit. Phones are protected quite well, so it is safer to keep a user signed on the device and maybe only a PIN or biometrics in front of the app to open it. The apps run in an isolated space, making it even safer.

Lastly there can be a performance benefit. Websites are written in HTML, CSS and JavaScript which is compiled on demand, while apps can be written in languages which are pre-compiled.

What banking or e-commerce sites don't work through a web browser? I've never heard of such a thing.

My bank literally forces me to use their godforsaken shitty app but wherever possible I use web over app especially for corporations (eBay, Amazon, YouTube, catch up TV etc). Like you & others said, reduces exposure to data harvesting

The banking app I use takes advantage of on device biometrics and in terms of payments, the secure element in my phone. I trust it more than I trust a browser.

My main worry is that if I lose my phone or it's stolen, especially if I'm overseas,I won't be able to access a lot of sites due to the 2 factor authentication. A lot of sites don't have alternative 2fa methods, only SMS or through the app. I've been trying to think of a way to access these sites if overseas and my phone gets lost or stolen, the only thing I can think of is to take my sim out and hide it, and buy a local sim when in a new country. If I need 2FA I put in my old sim, but for day to day stuff use the local sim. If the phone gets stolen / lost, I've not lost the 2FA capability.

A web page is more secure than an app, on theory. That’s because you can’t see the source code, tokens or whatever they use to build it. It’s called a ‘backend’. However, an app is insecure by default so there is a lot of tricks that must be done to secure it (for example Oauth 2.0 flow with pkce). And the software they use to gather the data to show (APIs) must be built better.
So… why then? It’s all about having the means to deliver the experience that the bank wants. Look at voyager app for Lemmy, for example. Why people want a iOS version when the PWA exists? Because it simply works better as a native app.