The problem with using live animals as dynamic characters for scripted scenes with actual choreography and such is that they don't know they're in a film. You can guide them to perform various tasks, but they never look as engaged as you'd want them to. Check out pretty much any actual live-action talking animal flick; their heads and eyes are always going somewhere else when they're supposed to be having an engaged conversation with someone right in front of them, and their body language and movements rarely ever actually convey the emotion or intent that their character is supposed to have. Any emotion just feels grafted on and distractingly fake. CG animal characters can be scripted to show all the species-specific behavioral ticks associated with various moods, filtering out all the random stuff that makes real animal "actors" seem out of character and distracted. Like, look at at how Shere Khan moves in my favorite scene from Disney's 2016 version:
https://www.youtube.com/watch?v=k4evCG3XBCo You can tell from his body language (posture and strut, the way his tail and ears flick at certain points to accentuate his mood, etc.) that he moves with ill intent, so you don't need the face alone to be particularly expressive. You're not going to convey that same mood and gravitas with an actual tiger, and you don't want to be around a real tiger when it does get that engaged with whatever it's approaching.
I understand the reasoning and respect Serkis' approach to motion capture work and character expressiveness, and this seems to be a passion project of his so I'm actually pretty sold on this outside of the creature design. (I was actually more looking forward to this take on the story than Disney's recent version.) It's just that otherwise photorealistic non-primate animals don't really mesh well with human-esque faces for me aesthetically, especially given the self-proclaimed "dark" tone of the film. This looks like what would happen if you made photorealistic models of
The Lion King characters or tried to pull an
Animorphs.
I think the CG creatures here actually look a lot better in terms of being more "present" than the Disney version's. The Disney version has well-detailed, anatomically correct models that look nice when they're by themselves -- even though you can still tell it's CGI, but of course you can; computer graphics of this sort are never going to look like real life -- but their non-presence becomes glaring obvious once Mowgli enters the shot. Not necessarily distracting to me since I'm used to looking at CGI and can suspend disbelief if it's good enough, but it is distractingly noticeable when he has to make physical contact with them.