Why does every celebrity have to snob someone on Twitter?

They don't.

Realize that they are người nổi tiếng 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 một giây of their life tweeting without eating, sleeping, hoặc even going to the bathroom. LOL
Poor them, they'd be dead bởi now.

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