A lot of stuff is neither on Spotify nor YouTube. One of my musical special interests is a very popular Polish rock band from the 80s and even they don't have their whole discography on Spotify yet, and I had to upload some rarer stuff to the Internet myself. It improved over the years though - just a few years ago you only had a few albums of their discography on Spotify. Now it's just one studio album missing - although most of their live albums aren't there as well and that probably won't change anytime soon; and not even talking about songs that got only released as singles.

Once you go "slightly less popular than one of the top bands in the country", stuff quickly becomes hard to find across the whole Internet, not just Spotify or YouTube, often even when CDs are still available to buy (which isn't a given either).