Required by FCC.
About 10 percent of total population of the United States suffers from hearing impairment. In order to provide access to televised content for the hearing impaired FCC (Federal Communications Commission) mandates closed captioning of DTV. HDTV as a subset of DTV is by definition required to carry closed captioning information.
Whether producers must deliver closed captioned air masters to networks is a matter of individual agreements. In some cases producers are required to provide captioning and in some cases they are not.