The lore is that the saarlac actually keeps you alive while digesting you. Like, it puts roots in that act as life support so it has a constant source of protein or whatever.

Sarlac that lost the ability to grow ‘roots’ and instead worked out venom and energy storage (fat and muscle) would outcompete and supplant the sarlacs still producing energetically-expensive, overly-complicated stasis bellies.

What happens if they momentarily fail to keep their meals alive, do they starve or become poisoned by the rotting meat? Do they have issues keeping a varied diet alive? How do they maintain a net positive energy balance after producing all that is required to keep organisms alive? Why not stun them and stash them, like wasps and spiders? Why not fatten up like -gestures broadly at all life-. Fat requires very little maintenance.

Could one, in theory, rescue one’s friends were they trapped in a sarlac?

Sounds like an organism that would quickly be out of business were there any competition at all.

As long as we’re coming up with overly convoluted reasons that a minor plot device from a fantasy space opera makes sense in a rigorous scientific way, why not assume that they were genetically engineered specifically as a torturous punishment for the Hutt syndicate? Bioengineering is apparently canon, so there’s in-universe justification.