>Probably because such a save can take considerable time, and doing it in parallel with GUI work is technically difficult.
not be 'that guy' but i'm a software engineer - and PI is incredibly impressive software, and autosave is not technically difficult. when i manually save it takes about a second of time, and incremental saves should take less and of course multithreading disk i/o in the background is not difficult.
even so, if even small interrupts were not desirable, it'd be relatively simple to check for idleness, and save if/when the user is idle. i think a lot of people take breaks or leave PI open on their systems for extended periods of time.