Hi,
The reason it doesn't happen automatically was that a lot of people probably didn't realise it was happening and so it was needlessly using disk space. I recall a few people mentioning it had got very large without them knowing it was happening.
To make it happen again you need to go to the Rendering prefs panel and check "Automatically save all renders".
As you've already found you can also use the Customise Default Paths settings in the File Saving prefs panel to make the renders save in a more convenient location. You change the "Automatic render output" item for that. I'm just mentioning that in case someone else comes across this :-).
Regards,
Jo