Fundamentally, this is happening because the quality of the song-writing isn't really all that important to the success of a pop star. Marketing, production quality, and sex appeal are more important. It's really not that hard to write a 4-chord song about breaking up with your boyfriend.
If song-writers think they should be getting rich off these popular songs, there is nothing stopping them from performing the song themselves. They don't do that though because they know they'll make more money letting Taylor Swift sing it.
The songwriting is extremely valuable, otherwise just write your own damn songs.
If it's so valuable, how can these companies find "hit" writers for £100 a piece? While it is entirely possible for something of immense artistic or technical value to not be priced commensurately on the market, I've heard these songs, this is not the case here.
In my view the same thing should apply to songwriting. The current standard is, you wrote a song if you wrote the melody or the words. Sometimes those are two different people.
If you added value in other ways, add your name to some other contract specifying distribution of proceeds.
The songwriting is extremely valuable, otherwise just write your own damn songs.
As an aside, "inherent value" is a mushy concept that seems to defy any meaningful description.