Can I play devil's advocate a little bit here, because I was really unaware that this ever worked for anything except indie artists on Bandcamp. So Bandcamp works for them or for discovering new music obviously.

I didn't know artists ever sold digital music on their websites, but that does make sense, so I checked - if I google Taylor Swift and go to her website, there it is, digital music purchase. Great.

I went to U2's website, and the only music I can buy there is vinyl. I don't want vinyl, I want digital. You can buy merch, but I'm after music, not merch. Looking further, there's all sorts of galleries and information about each album and song, but you still can't buy the music.

Other mainstream artists I googled didn't even go that far. Googling them brought up a wikipedia link, social media links, tours. All stuff I don't want. Now your list has "contact artist via social media" - setting aside the fact that it's unlikely a popular mainstream artist will even reply to anyone at all about anything, this is a real point of friction. I don't want to have to contact an artist to find out some alternative way to get their music. If I'm buying something online, there needs to be some way to buy it online and ready to go. If we have to wait a couple of days or weeks for a reply that may or may not come - the process failed.

If I had to guess, they would probably say something like "it's on spotify".

So yes it probably is a supplier problem, but it seems to me that this is happening for the majority of popular artists if a majority of music people like is mainstream. I assume if you like the majority of indie music then that's probably not the case.