Cats Like Their Autonomy
Feral (wild) cats have their own territories and are responsible for their own food, water and safety. This autonomy and feeling of self-preservation is also seen in domesticated cats to a certain degree. Some people may even call cats aloof or unfriendly because of this.
However, for all the times you find your cat alone doing “cat things” (perhaps plotting to kill that evil dust bunny lurking in the corner), there are plenty of occasions your cat is quite social.