Why do actors leave shows because they're not making "enough" money?

(ex: Paula Abdul leaving American Idol because she wanted more money and didnt want what she was offered) and i'm sure @ least SOME of them do it because they feel like after a while they DESERVE more, but what the hell, they're making millions and billions of dollars in like ONE paycheck. y cant they be happy with that & take into consideration the fact that normal ppl only get hundreds of dollars in their pay?