Not all.
Just like with a lot of things. Sure you could do a thing better, faster, more efficiently on a PC, but some people just don't care when 80% is good enough.
I log in to transfer money, to take a photo of a check to deposit it, to check my balance.
All of that is fine on a phone screen. Actually, it's a lot easier to take the check photo.
And a banking app is a whole lot more secure than a browser tab running extensions that might get hijacked, on a desktop OS whose architecture allows this like widespread disk access, keyloggers, etc.
BTW newer mobile phones offer "desktop mode" (the Samsung Dex, and what came to AOSP), so you can attach them to a TV.