@alina I can describe my view of it in the hope that it is helpful, though of course I don't know if it's the same reason as you have:

Because 'social media' as we practice it today centers entirely around the self, it is egoistic and essentially a popularity contest... community building is an afterthought and it is highly individualistic, and so instead of bringing (even like-minded) people together, it just locks everybody in a permanent battle of "showing you are better than others by some metric".

The exact metric varies by platform and subculture but there always is one on profile-oriented 'social' media, even this one, and it isn't really very 'social' at all.

Does that sound about right to you, or were you thinking of something different?