I can answer that with ease

Exceptional players have no need to sing, their skill shines through their play..

Secondly, when you are a great player, there is more involved then playing 2-5 chords, or following tracks.

They have mastered their skills..

I believe I have only really known one great player that sang .. He was also an average singer.. (Brent).

I do know many "singers" in the next group that play to back themselves... I don't believe many if any are great players..

Personally I rather listen to a great instrumentalist ( and appreciate the skill level, then a "pop" type singer. The exception would be throwback to old groups that I grew up with)