Why isn't Zack and Cody closed captioned?

I think because most people today with hearing problems have a hearing aid.

Actually it should be closed captioned, due to the ADA, Americans with Disabilites Act. All network programming is to be close captioned due to this law. If it is not, you should contact the network and inform them one of their shows is not close captioned. It has nothing to with people having hearing aids or not. Hearing aids alone do not guarantee comprehension of auditory stimuli, for all Deaf/Hard of Hearing people.