Whilst you are right that it is not absolutely necessary for a CEO to code - it *is* necessary that the technical role that drives it is filled by someone who is in sync with the CEO or that they are a person who is able to accurately convey and then direct the development of the product.
Put it this way: A CEO that codes is cheap for a startup, but if a startup has the capital and access to the talent then the role can be filled by more than one person.