Snakes are escape artists, even very large snakes. Any opening in an enclosure and they will squeeze through it, and they can climb over remarkably high walls. These animals are also tropical and need environments of specific humidity and temperature conditions, that's another potential issue with a more open habitat. Zoos certainly put the bigger snakes in bigger exhibit enclosures. I've even seen very large ones the size of a house where there is a glass wall people can file past and see the snake in a sort of rainforest setting with plants and an artificial pond or river. Some of these exhibits can be quite large. But they are always completely enclosed, glassed in, and covered over in so the snake can't climb out or squeeze through a gap. I have never seen a zoo that has snakes, even very large ones, in an open-air type habitat the way they might with their tortoises or even komodo dragons. And of course a walk-through style glass habitat would not be a good idea for humongous snakes that have been known to at least occasionally attack (and even kill) humans.
The uniform size of the exhibit enclosures is the less realistic element in planet zoo, since they have the yellow anaconda and boa in the same sized exhibit as a tarantula. But I am guessing that having different sized exhibit "cages" would make things harder for the programmers.
I think it would be cool, though, if it were possible to design completely enclosed and covered larger exhibit type enclosures for the huge snakes, or for flying birds. I'd also like to maybe see exhibits included for small mammals that wouldn't work in a normal "open" large habitat.