Those are two different things. Recording artist does not always equal songwriter. So how much should the songwriter make? The recording studio? The audio engineer? All the other people involved in creating the recorded song? Now that it's made, how do you get people to know the song exists and want to listen to it, much less purchase it?
The reason compensation isn't a settled thing is it's a very complex thing to answer.
The simplest possible answer is "the artist sets their own price" - assuming they just DIY'd the entire production, advertising, distribution, etc themselves. But that is so much work that they would need to already have an income stream to give them the time to do it all, not to mention all the non-music skills if they're not paying professionals to do the rest.
If they're not just going to play at the local coffee shop, or bus from city to city barely making enough for gas and beer, they need some way to professionally produce, mass-market, and mass-distribute their songs. It's not feasible for most musicians to do this themselves, so there exists a music industry to do it... which gives them all the cards... letting them set the price, and contract terms... which are often unfair. That's what happens when an industry is given the power to exploit people: they do.