@Left Hand Percherons - I didn't say they were incapable of digesting it, but they can't do it efficiently yet.
And about the young horses in the wild. They get enough nutrients to survive, a lot of the times these young horses are pretty scrawny though. For them to get enough nutrients for sufficient growth their diets need to be supplemented with a high percentage of concentrates.
It seems like it would go against nature but it doesn't. This is what research has found. If you would like to look into it yourselves there are a lot of equine nutrition research papers you can search at Google Scholar