Spotify (and I assume others) doesn't exactly serve a random playlist, but one that favours/returns more revenue back to them. So older releases, "burned out" songs and so on. Create a playlist with 10 songs, add 8 before the 90s (e.g. Cure, U2 etc.) and add some newer ones (after 2000 - e.g. Radiohead). You'll be living in the 80s for most of the time...

If you search the Spotify forums, there are a few posts on the issue with no official answer from Spotify whatsoever.