How do you know this? Of course there are lots of reasons for why they’d want to enforce minimum browser versions. But security might very well be one of them. Especially if you’re a bank you probably feel bad about sending session tokens to a browser that potentially has known security vulnerabilities.
And sure, the user agent isn’t a sure way to tell whether a browser is outdated, but in 95% of cases it’s good enough, and people that know enough to understand the block shouldn’t apply to them can bypass it easily anyway.