The only thing other CAs do (after EV certificates stopped being a thing, as a sibling commenter already mentioned) is to take more money from you than Letsencr

The only thing other CAs do (after EV certificates stopped being a thing, as a sibling commenter already mentioned) is to take more money from you than Letsencrypt, in exchange for longer validities and historically some other concessions (although the browser forum has been clamping down on that, for good reasons). In other words, if the bank is following best security practices, they're fine with Letsencrypt; if they don't, they might need somebody else.