Erm, because you still need to log in to validate that you're authorized to read the feed.
Many financial institutions make available transaction logs in CSV and other machine-readable formats, usually labeled "Export to QuickBooks" or something similar.
Plenty of other ways to do auth out there. Heck, if they just emailed me data that would be an improvement.
The problem with CSV, QFX, and similar is that it generally doesn't contain all of the data available from the website, like check images, statements, etc.
I'd love to be able to script up a really RSS grabber that dumps directly to disk and other scripts that do all this automatically.