Larian: "Oh I know guys, let's build the game with a fucked up ruleset that nobody in the history of RPG has used....and let's troll the player while at it"
You must be quite young and unaware of RPG history.
In Dungeon Master, you had to eat and drink.
When you slept to regenrate (mana), you became hungry.
You had to constantly care about food and water.
And when too hungry you started to loose health slowly.
Same with the quite recent Legend of Grimrock from Almost Human.
Closer, in Baldur's gate (and all AD&D games), there was no auto regen either. Only during fast travel but once on a map you had to do with the memorized spells.
And if you slept to re-memorize spells you risked a random encounter.
Even closer in Skyrim (I known everybody does not like it, but I do and its sales proves I'm not the only one), by default you do not regenerate health either. Only mana.
I was surprised in the beginning of D:OS (and happy to have decided to take a healer with me right at the start).