My information here may be outdated, but when I was in Seoul for awhile, it wasn't limited to just banking apps, many services had similar requirements for specific plugins, even requiring Internet Explorer 11 and a bunch of plugins for that.

I remember trying to get tickets for an event, and it was not possible within MacOS at the time due to the various Windows only requirements. I remember even having to re-download another version of Windows 7 as Tiny7 had various Windows Services removed that for some reason the plugins/apps relied on.

My cynical guess is that the plugins/apps include user data/telemetry that the companies get a cut for, but of course this is just supposition. It's entirely possible it's just some liability thing that has become entrenched in Korean IT, who knows.

But the practice was everywhere.