That is pretty much how music industry work though. Unless you are also the singer, nobody care if you write the song. And since they all sing by Miku, most people search her music based on the composer so they actually have more exposure than normal composer.

This was among the reasons why Wowaka left the vocaloid scene, he felt Miku was overshadowing his work. His songs were "her" songs in a sense, he ended up forming his own live band afterwards. He did make a final tribute to Miku on her 10th anniversary.