I would imagine it's a realism issue. Real zoos don't tend to plant edible plants in animal exhibits because managing how much an animal eats is an important part of zoo husbandry. In saying that, real zoos also don't tend to have plants from all over the world because they're hard-to-impossible to actually maintain; typically they'll use local plants that look like they could be from, say, a jungle. The exception of course is when they put in those big greenhouses and whatnot.
I had hoped that Planet Zoo might address this prior to release, but it's not a big deal anyway. Instead of plants being from a specific place, we could have just had biome-specific plants that are "generic".