One, because "success" often comes with more responsibility and less choice. More people depend on you, in more ways, and you can't let them down if you want to keep your "success".
You might find it annoying to unfollow your successful acquaintances, but imagine how it feels to be on the other end, with multiple investors threatening to pull funding and kill your company when your "social media presence" is waning -- and you can't even express your frustration on Twitter because that would be company suicide.
Secondly, it gets worse because your standards of comparison change. You compare upwards to people with even more success. And, since "success" metrics are exponentially distributed, so is the gap that you feel between the people more "successful" than you.