Why does every celebrity have to snob someone on Twitter?

They don't.

Realize that they are Знаменитости and there's not much time on their hands. If they respond to you, they have to respond to everybody else. And if that happened, with their popularity, they'd respond every секунда of their life tweeting without eating, sleeping, или even going to the bathroom. LOL
Poor them, they'd be dead by now.

Now if Ты were a celebrity, Ты couldn't respond to all tweets directed to you. It's too hectic.