Wild horses have thousands of acres of land to roam. They move 10+ miles a day, which wears their feet down. They can browse on all kinds of grass, bushes, trees, other plants to find all the nutrition they need. In bad weather, they can seek out shelter in trees, gulleys, depressions in the side of a cliff, etc.
A domestic horse lives in a confined space, usually with other horses. They eat on the ground they pooh/pee on, even if you pick it up, the parasites and bacteria are still there. We give them hay, which is only eaten in the wild in the winter, when the grass is dead... Because of said confined space, horses move around less, so their feet grow longer. They have less access to fun things to munch on. They have more confrontations with other horses, which can lead to injuries, spread of disease, spread of parasites, etc.
A well taken care of domestic horse has an average life span of 20-30 years old, barring any accidents. The average life span of a Mustang is only about 10 years old... If they get hurt or sick, they become lunch.