A lot if it is selection bias. Humans prefer animals that show those traits. We instinctively understand how they are thinking/feeling, and that makes us more comfortable with it.

It’s also worth noting that complex mental pathways take a long time to evolve. Nature tends to play with there tuning, rather than strip it out when unnecessary. Most solitary creatures had ancestors that formed groups. There’s no reason to risk breaking useful instincts. They just get overriden by newer ones.