That's not an equivalent analogy. It's the exact opposite in fact. The equivalent analogies would be:
Why don't screenwriters just film their own movies?
Why don't composers just form their own orchestras?
Why don't architects just build their own skyscrapers?
The screenwriter example should make it obvious why these song-writers' complaints are baseless. A movie filmed by amateur videographers with no editing experience, no acting talent, and no marketing is not going to achieve mainstream success no matter how good the writing is. Meanwhile movies with terrible writing rake in huge numbers at the box office just because they have big name actors and top-of-the-line production quality (ex: the Avengers). This is because the script isn't particularly important to the success of the movie. Hence why the script writer doesn't get paid a large percentage of royalties.
If the quality of song-writing was a key determinant in the success of a song, the song-writers would have leverage to negotiate better royalties. But it isn't, so they don't. If you don't like it, stop listening to pop music. There are plenty of struggling artists playing their own compositions in dive bars on friday nights that would love your support.