That was the brief moment when the iPhone was the only smartphone to target and was 320x640. Or when the majority computer screens where 1080p at most and the browser would be on more than half the screen estate.
But for your bank for instance, if their UI is optimized for a 6" diagonal window, they'd probably expect you to adjust for that instead of them trying to be perfect on every screen combination that could happen on earth.
That's also how I see many support chat apps' choices of spawning a popup when running on a desktop, to reset any browser size the user was trying to use in the first place (users are still free to do whatever they want with the popup, but it's a good indication of what size it's supposed to be)
This is possible by abstracting the layout engine (generally to match a web browser) and having UI control running in transpiled JavaScript.
In more practical terms, this means its easier to have one team managing your website and mobile app, reducing the number of specialist roles and minimising feature disparity. Generally it saves on duplicate work as well and makes timelines easier to manage.
For banks specifically there are a bunch of wins with respect to only having to ensure legal compliance of one application; this applies to banking laws - where certain information must be communicated at certain points, and to public accessibility laws. Most of these frameworks have a lot of tooling around language support and accessibility integrations.
So even if these experiences are inferior, it is very much worth it to the providers.
One interesting usage I've seen is especially younger people, who have a debit card which doesn't allow an overdraft. They then keep their account at or around zero and only transfer the amount they need to the card account before every purchase. That usage is supported way better than my attempts at managing saving, paying large bills or keeping track subscriptions and other spending for the past week.
I also have a separate savings account, and one for my cottage, both which get scheduled transfers from my "main" account.
When my debit account is low I transfer $100 or so. Allows me to have a sense of spending that cash gave me.
Most people have at least two. One of their incoming paycheck and one for their debit card. Most have more.
There's an accessibility element here as well. It's not possible for everyone to use a touch screen.
The issue I have is that the mobile app is already so dumbed down that it's pointless and I don't use it. When you have limited screen space you need to decide what to show first. For my bank it's the status of a MasterCard credit card I don't own on the first screen of the mobile app. Next is some pointless overview of my accounts. I say pointless, because all context has been removed, in favor of massive amounts of white space.
Now they want to replicate this interface, but for larger screens. Most of the screen is white space and you have to click on everything to get details, details that would fit perfectly well, even on a small laptop screen. Also nothing is obviously click-able, because why would you add visual clues that just taints their beautiful white space.
For the most part I think that companies would love to ignore desktops and large screens. In some sense they may also be afraid of presenting users with details overviews, either is to make everything seem more friendly, or to discourage usage.