I used to think the same, but these days it seems like most songs from my favorites/liked list are no longer on Spotify, as I hear the same 10 or 20 songs over and over again when I have it on random play, and when I manually try to go through my list it’ll skip over songs and not let me select them.
I guess the competition with the other music delivery companies is coming down to certain companies have exclusives for certain songs and artists.