Why does every celebrity have to snob someone on Twitter?

They don't.

Realize that they are セレブ 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 秒 of their life tweeting without eating, sleeping, または even going to the bathroom. LOL
Poor them, they'd be dead によって now.

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