The answer is so simple. I can't believe none of you have thought of it. Kabuto does not have a seal. The cursed seal is like steroids, it gives a short term boost of stregth but overall it crippples a persons development. The sound 4 were given the seal by Orchimaru to make them his bitches. They can't act indepedently on their own because of the seal.

Kabuto is much more useful to Orchimaru without a seal because he is a strong ally and great indepedent thinker. Orchimaru respects Kabuto and finds him to be more useful when he works autanomously without being binded to anything. Thats why Kabuto most likely does not have a seal and will probably not receive one unless Orchimaru wants him to be his bitch.