natural exhibits

In pretty much every Zoo I visited, even modern and big exhibits, great apes have no acces to trees, they are either weired, to stop the apes to climb on them, or are not even present. My question is why. (With few exeptions like the Gorilas exhibit in Apenheul) Zoos don't give them acces...

Just wondering on your opinion, do you think all exhibits need to be naturalistic? I personally think that while some exhibits in zoo should definitely be natural, it is more important for the exhibit to be functional for the animals, visitors, and keepers.