Why is WordPress so easy to install, so painful to update?

I don’t think there is a better open-source blog engine on the market than WordPress, but why is it so painful to update it? You have to back up the database, download the files, copy over the config file, themes, uploads, and any other stuff you need to the new installation, and then rename the folders quickly so the new one fills the space of the old one.

Why can’t we just get the files that have changed downloaded and the database updated by pressing a big blue “Update” button?