Yeah, it seems that devs didn't think players will be making huge habitats.
Which is strange, because some of the plants and props they give us don't really work except in large habitats, and even if you make an elephant enclosure a fair bit above the minimum size indicated for the animals, a shelter structure that is sufficient to grant them all hard shelter never really fits. I have to make huge elephant enclosures to give them enough hard shelter.
Also, the monorail and safari adventure rides are designed for larger, more natural habitats. At least that's how I've seen them used in real zoo "safari parks
Hit box issues, I guess.
But some of the habitats that show up as non traversable by staff in my zoos are not any larger than ones that show up as traversable. Sometimes they are in workzones that are further away from the entrance in a given zoo, which Kolihokoli also mentioned. I don't know if placing a second entrance in a zoo affects this or not. Will have to experiment.
One that showed up as non traversable was in a habitat that was built on a hillside, so maybe elevation affects this. I was worried the keepers wouldn't be able to get up the hills to fill feeders and clean, but they actually were able to just fine. The entire enclosure showed as non traversable, though, not just the elevated parts.
It's not an issue that makes the game unplayable or anything, but it's handy to know whether or not there is a part of a habitat keepers can't access for real.
Grampers is correct about the design of habitats for access, but this issue refers specifically to habitats that actually are accessable to staff but show up as completely non accessable.