According to this post, there are albino exhibit animals possible in the game.
Just a simple list of all white animals Planet Zoo can offer! DLC Animals have their own list. ...
steamcommunity.com
I personally have never seen one for sale in the exhibit animal trade lists, and the person making this post says they "haven't been found yet," so I don't know how they knows they are available. If they really are, they must be incredibly rare. Maybe they get snapped up so fast the odds of seeing and purchasing one are very low. And since one can't sell the exhibit animals one has bred to other players the way we can the larger animals in the game, there wouldn't be the same concentration of the genetic variant in the population once it shows up. I assume the white lions and so on were rare when the game first launched, but once everyone started breeding them and selling offspring that at least carried the albino/white/dilute alleles to other players, they became really common.
Mind you, I haven't seen an albino Tapir or Orangutan for sale in the game either, so some of the white variants must be much rarer for habitat enclosure animals too, or maybe players are simply less interested in breeding them when they do show up.