I think (and this is just my personal opinnion) that IF there would be “CORE” or “Normal” setting it would easily lead huge complains from people who think “I have played ALL games atleast Hard / Normal”.
Now when they try “core” or “normal” P&P rules it came as huge shock that one critical hit might just kill they PC. And it can happen quite often. So “game is not balanced, it is unfair”.
If I remember it right developers actually changed names of the difficulty sliders in Kingmaker so (“normal”) player were happier because game was bit easier but still did not read something like “less then CORE” or “Less then Normal” P&P ruleset.
After all most people do not really think that pretty much every area and encounter should need thinking, buffing and use of allkind of magic and strategy. Not just “auto attack”.
All in all, I am not 100% happy with no “Core rules” difficulty button but if I can easily still change setting so that they are as near core P&P ruleset as possible then I do not complain. Specially becouse I do realize that by making it that way game sell WAY more. What mean in long shot that I will get more Pathfinder (computer) RPG’s to play with.
Heck, I even played this Alpha test using recommended “normal” game setting. Take still nearly 70 hour for me and I did not had to make reloads because of dying or so. So yes, it is definetly easier but if you have not played P&P games or even much of computer RPG’s before it is “hard enough” for new player.