Uhh, thats because Planet Zoo is not "the wild"?? You're making a Zoo, not "the wild". A zoo is a simulation of an animals wild habitat, but it is not "the wild".
Animals eat/graze regularly, especially the roaming ones, like the zebra, or wilderbeest. I doubt an enclosure with grasses can keep up with such numbers of animals grazing on them. As for the poo, sure, you can just leave them in their enclosures, sort of a fertilizer, but dont expect that poo to deteriorate in a matter of minutes. In the end, you're likely to have an enclosure ridden with dung lying around.
what you said kind of makes no sense though....
''Uhh, thats because Planet Zoo is not "the wild"?? You're making a Zoo, not "the wild". A zoo is a simulation of an animals wild habitat, but it is not "the wild". ''
Just because its not the wild, doesnt really mean they shouldn't eat the grass? that is kind of like saying, that if you keep horses in a field, they are never going to eat the grass around them, an only ever eat the hay that is left around... in dublin zoo, almost all the herbivores have grass in the enclosures that they can graze.
i mean, as is, a lot of zoo's will try to include natural ways animals eat, like hanging fruit for monkeys to get, or having meat sacks tied up for carnivores to try to get at, grazing is the same.
also, if you where to make one HUGE enclosure, herds move, giving grass in other spaces time to regrow, its not like they should all just stand still in one spot.
im pretty sure OP was thinking of making one massive enclosure like a wildlife reserve, hence the grass re-growing an such an the mention of grazing, if its a big enclosure, there should be more than enough grass and plants to do animals, especially if plant and grass growth is a thing
Edit:
id also like to point out, Cattle eat up to 27 pounds
per head
per day.
Zebras eat 20 ponds per zebra
a day
and yet... we keep a TON of cattle on land, and they dont run out of grass?