It’s a bit of both. Diet, volume and biology.

Humans mostly absorb iron through the duodenum, which is a very short bit of intestine near the stomach.

Herbivores, on the other hand, have either massively complex systems of stomachs, chew their cud to make nutrients more absorbable, or letting food ferment before digesting. The latter also works for humans, if you like fermented veggies.

Of course, diet also matters. Humans don’t eat all that high iron foods, but grass is a cow’s main food source and it’s high in iron.