I guess there is some subconscious jealousy involved. As an instrumentalist one might think, music can be done by producing the right notes, tempo, pitch, rhythm, expression etc. - the “technical” aspect of it.
So why is a singer more successful, if some of these aspects seem to be lacking??
They might have the ability to reach the audience in another way, make them cry - for whatever reason 
Talking about opera: very simply said, drama and theatre started with just spoken words. Later music was added, as it seemed to work well. In the mean time, music has sometimes taken over the drama and got “too” important… [end off topic]