I use Exchange, and my OST files are still under AppData\Local\Microsoft\Outlook
Why would the PSTs be under Documents? It isn't a human-readable file.
"I'm reinstalling Windows and I need to backup my email so I still have it after the reinstall."
<proceed to walk a non-technically inclined user through the process of unhiding folders, possibly needing to cover the concept of user profile directories, and other additional things depending on his configuration>
XLS files aren't human readable either. Nor are JPEGs, PNGs, GIFs, MP3s, AVIs, or PDFs. Like PST files, they all need a program to open up the data file and present the information to the user.
And like XLS files, JPEGs, PNGs, GIFs, MP3s, AVIs, and PDFs, Outlook's PST files need to be in a place that's accessible to an end-user who has no interest in earning an MCSE certificate just so he can back it up when he needs to reinstall Windows or migrate
to another PC.
OST files can stay where they are because they're not used directly by the end user and serve only as a cache. They have no need to be touched, located, or backed up by a non-technically inclined end user.