It looks like it depends on the ownership situation of a particular song.

Some of the songs are on recordings that he is the sole owner of. Those songs are available for download or streaming on that website.

Other songs that he wrote have more complicated ownership. If the song was commissioned for something (e.g., the Sesame Street and Electric Company songs), then he is not the only party with ownership, so those songs do not have recordings. Where he wrote words for existing tunes, the same situation applies.

IOW, some songs have recordings, others do not.

BB,
NightWing