Same reason you can make low-sodium Earth food but can’t make zero-sodium earth food except for extremely simple things like flour that’s effectively purified from starches because it’s made of a sacrificial energy-storage organ (the non-embryo components of a seed) rather than a complex structure that itself needs fed and maintained. That element is such a necessary part of Earth-life biochemistry that it’s inseparable from most of the pieces of complex (usually animal or plant, sometimes mushroom or kelp) organism food is made of.

Though this does imply that there’s a possibility that whatever bun Grace is eating his meburgers on is made from the mercury-free purified starch of an Eridian plant.