Because updates break the jailbreak. If Apple actually had a sanctioned way to let you do what you want with the device you bought there would indeed be very little reason to not update.
I don't know where you live, but where I am most banks insist on proprietary 2FA via their own app. Only (some of) the fintechs allow TOTP.
Yes you can get your codes via SMS, but I thnk we all know what a security mess SMS is. I would rather have the bank's proprietary app since at least everything will be going over HTTPS and also eliminates most of the vulnerabilities associated with SMS just by virtue of being an app.
Or why have a smartphone in the first place if you're not going to put any sort of meaningful apps on it. ;)