The pack animals typically can't eat enough to entirely subsist off of local plants. Plus any time that you have to let the animals graze is time that you aren't marching.
"Wagons are more promising. A big wagon pulled by two horses can carry perhaps a ton (1000kg) at maximum (in practice many medieval wagons capped out well below this and were pulled by four horses), but now we have two horses and a driver to consider...Their nutrition requirements are too high and so they require feed, at least some 4.5kg of it per day assuming local grass is available along with time to let the horses graze it (during which the wagon is, of course, stopped). The Romans seem to have allocated around 7kg of barley per day per cavalryman for their cavalry"
EDIT: Ah, I did not realize before linking the acoup blog that the article is actually in response to that same article.
ACOUP had a good writeup on it https://acoup.blog/2022/07/15/collections-logistics-how-did-...
"Wagons are more promising. A big wagon pulled by two horses can carry perhaps a ton (1000kg) at maximum (in practice many medieval wagons capped out well below this and were pulled by four horses), but now we have two horses and a driver to consider...Their nutrition requirements are too high and so they require feed, at least some 4.5kg of it per day assuming local grass is available along with time to let the horses graze it (during which the wagon is, of course, stopped). The Romans seem to have allocated around 7kg of barley per day per cavalryman for their cavalry"
EDIT: Ah, I did not realize before linking the acoup blog that the article is actually in response to that same article.