Any reason for why is this disabled with each new session?
Forgot to activate it and just lost 3-4 hrs of grooming work (due to a simple hide/unhide object). I feel like going on a long rant, but now I have catch-up to do…

It's a lamented issue by many ppl.
And it can lead to a nightmare scenario when it's combined with Houdini crashing and corrupting your project file (it happened to me a couple of times, but I had backups luckily)

This is the response I received from the Support:

The reason why we don't make this option stick is because we think there is a fine balance to what users want when it comes to opening/saving very large (enormous) files which might interrupt their work. The way to have the autosave always on, is to put the hscript command “autosave on” in one of your 123.cmd or 456.cmd files.

Of course you are welcome to write the Support to share your experience.

Great, now I have to fiddle with scripting an google searching instead of continuing my work.

That's a rather perplexing explanation - the inconvenience of going through a long(ish) auto-saving because you forgot to toggle it off is on par with losing hrs of work because you forgot to toggle it on?