Because they incentivize it. Some banks are better at incentivizing it than others. My bank for example, allows the highest daily limit (by a factor of 5x) if you use the app. Online banking has a lower limit, and cards lower still. I don't appreciate them holding my own money hostage, but the sad reality we live in precludes me from having enough remaining mental bandwidth and effort reserve to commit it to fighting against it in such an empty and unwinnable battle. Money is a scam anyway.

The only thing I use my bank app for is to deposit checks. It can't be done from the desktop because it needs the phone camera to take photos of the check.

I don't know how online banking access is around the world, but here in brazil, they made it completely impractical to use from computers by applying artificial restrictions. Some payment institutions doesn't even support access from computers anymore. Meanwhile, accessing from the phones has been made easier and less restrictive, so basically everyone has to do banking from the phone. It has even become a popular thing to have a separate "banking phone" to use at home, but many people can't affort that.

In the US, I largely agree with you. Or use a website from a mobile browser. Different story in different countries where a smartphone might be the only compute the average person has, or where state services are tied to a mobile ID or bank app.

Not saying that should be the case, but if the choice is between running niche FOSS apps and removing yourself from societal benefits structures, I know what most people will pick. That's the real danger of allowing one company to own an entire ecosystem and have enough power that they have conversations directly with governments about their people instead of with their people.

I have a concrete and very stupid example.

We got a large gift card as an incentive to renew our lease at our current apartment vs moving. The format they sent it in had no physical card and would only work on either online stores or through a service like Google Wallet AKA a banking app on a mobile device as you mentioned.

So to get groceries while waiting on a tax refund (thanks crappy American economy and taxation methods) we had no choice but to connect and use it that way.