Yeah, it's more structure and content than special effects.
I still don't think CG's at a place where it can convincingly pull off photorealistic living animals, Avatar's a stunning looking movie (and nothing's come even approaching that close since that came out in, what, 2009? Jim's planned sequels should look pretty effin' amazing), but even that's fictional creatures. Real animals are tougher, as he have a reference point and we know what they should look like. Uncanny valley problem.
But moreso than that it's just like...you have to sacrifice something, whether it's a show or a movie. The movie comes with the problem of how you condense a plot taking part in smaller little "event chunks" over 53+ books, some of that's filler but still a whole lot of it's necessary to convey the story. And conversely, with a TV show you're sacrificing budget/visuals, putting a cap on what's possible - also the problem of if it's a network show it'll be majorly neutered, and if it's cable you're shrinking your own potential audience.
Even if you can somehow convince the suits to be lenient with the budget and let you go nuts with it, the narrative trouble still trumps that issue big-time.