On Spotify, I have seen that the majority of the time when there is a featuring artist, they are put in the song name but not as an artist. Why is this?
I think it's just a matter of making the categorisation more clear and easy. When it's a single song feature on an album, it would be technically be creating two artist entries in whatever database is housing it for the sake of say, one song.
If it's a collaboration that spans a whole album, often the artist title will reflect that. (e.g. Edgar Meyer & Chris Thile)
That said, it's common for movie, television or video game soundtracks with multiple composers to feature in the artist heading and the soundtrack to just be attributed to Various Artists. YMMV, but say, a pop or rap song featuring another artist is best listed as (feat. Joe Bloggs).