Spotify wants to offer you all the world's music, and they're working hard to make this happen. However, sometimes agreements can’t be reached with the artist or label or a change may happen in music ownership.
Spotify adds thousands of albums and artists are added every day. If you can’t find an artist you’re looking for, they may appear (or reappear) soon. In the meantime, feel free to contact the artist or their label to ask when they plan to be on Spotify.
In the meantime also check the available live albums of Paul McCartney alone and with his band The Wings, as he is doing some Beatles songs there as well. To convince their label Apple to offer their catalogue on any streaming service is something that even the richest hardware manufacturer Apple has not been able to do for Apple Music, although they offer the only download store which has these albums in iTunes.