I watched a guitarist on YouTube discuss some concept about harmony, and he played a note and sang an interval above. He prefaced with "now, I am not a good singer but...", and proceeded to sing notes with excellent pitch accuracy.
Obviously there's more to singing than just hitting the right notes, but hitting the right notes is a big part of it! Musicians who have been playing for a long time, or went to college, who have developed an excellent musical ear and relative pitch; are they automatically good at singing?