I mean there's an excellent and obvious answer and that's cost benefit when it comes to QA. Anything transactional, banks in particular, want to be 100% sure that end user experiences are doing exactly what they're expected to at all times. No one is being served incorrect information or is improperly served terms or disclaimers that they can use a leverage in a lawsuit. The tech teams likely have an explicit support matrix of browsers to test against and anything not on the list is considered untested and unreliable. They can't legally indemnify themselves against defects.

Interesting point on lawsuits! I never thought this way (maybe because lawsuits sounds like a very American thing, and I'm in EU)

It’s not so much for happy experiences, as it is to place bounds on what the development team is asked to do.

They may have spotted a bot using this UA and deduces it's a pattern