Cause he's a winy little b*tch.
"Oh nobody loves me; I don't know who I am; I love my glasses; my main motivation is to 'find myself' so I should summon everyone from the dead, avenge someone I constantly thought about killing myself, and kill everyone to accomplish 'finding myself', that makes perfect sense." STFU and get laid!
His character makes no sense, his powers came from...? He was supposed to be as strong as Kakashi in part 2 but got owed by 10 year-old Naruto. He was a completely useless character until he magically gained power so that Kishi could use Edo Tensei to bring back all of the old ninja for Kishi's self-loving, own-fan-boy, totally ridiculous All- star ninja war.
Kabuto is a useless, winy punk-a55 b1tch-n*gger who was forced into the storyline to make writing easier for Kishi.