Most likely that is what has been licensed for them to use. Some content gets restricted by the copyright owner/studios and held for use for their specific purpose or they only license certain content or in some cases what streaming services are willing to pay and have negotiated to utilize. Some content is in public domain and can be freely used, some syndicated, etc....it really just depends on the content. Missing episodes or only certain episodes are fairly common especially for free content/ad supported type channels, such as Pluto, Roku and others.
