A save feature is not an unreasonable request. In theory you can get a feel for the game mechanics and test out the game without needing to save, so the lack of saves isn't a big deal. I installed the demo to see if a system could play it (doesn't quite meet minimum requirements), and uninstalled due to sluggish responses, without even thinking to try to save/load. However, I can certainly recall other games where there were issues with the controls or questionable gameplay, so that I needed to finish the demo to allow enough time to get a feel for the game. In those cases, lack of saves would have caused a problem.
Even assuming there are no tough battles that would have you starting over after 20 minutes if you die (or does the demo auto-save kick in during the same session?), lack of saves means you can not reload to try quests different ways, or see how NPCs react to different conversation choices. You can go through the demo multiple times, but that isn't quite as immediate.
With one game I installed the demo, but then got busy for awhile and forgot about it for another while. When I got around to trying it out again a little over a month later, it had expired (didn't see anything about a 30 day expiration when I downloaded or installed it). I emailed customer service to see if I could get the time reset, and they said it expired so hackers couldn't use the demo to help bypass the copy protection in the full game; I don't know if it got cracked (wasn't that popular a game) but if so I'm pretty sure they didn't need the demo to do so. Their suggestions (re-install, play under a different user account) didn't help (had already tried), so if I had not been able to try the demo on a different computer, I would not have gotten the game (twice even; one for a Christmas present).
Anyway, presumably Larian and/or the various publishers have some kind of study that shows disabling saves encourages more people to buy than it turns off from playing, or prevents some amount of piracy, or something.
Oh, and welcome to the forum.