Why has this animal not integrated into society and learned common language?
Because it already has - the society and common language of its species.
Regardless of official rules, I'd expect at least a few animals in any high fantasy setting to parallel humans in many ways, with their own realms, hierarchies, codes of conduct, and channels of communication. These socieities would be distinct from those of humanoids (or humanoid-adjacent species), and based on different biology, histories, and principles, and by their fundamentally different nature, they'd be nearly invisible to "ordinary" humans. The magical ability to speak with animals would just offer a window into those realms.
Granted, I wouldn't expect them to be all that complicated, but conversing with animals does seem to be a common fantasy staple.
Also, the following seems relevant. Maybe. At the very least, I'm shocked and saddened that no one else has posted it.
