K-Pop is often so complex you need a variety of people and talents involved to shape a song. I'm sure there's one 'senior' writer maybe for a song who creates the overall vision for others to feed off like Teddy or Brave Bros.
Someone can be great for writing a melody or a hook, but someone else will flesh that out with some great sounds, beats, change ups, drops etc that elevate the track to what you hear on release, and the song would not have been as good without that input. I think Soyeon does a fair bit, but like someone said she's 22 and a full time idol it would be crazy if she could do it all herself at this stage if at all.