Honestly
I think like many people I have talked to it is fear of singing. Look at it this way, sing for you ... forget what others think but sing to make you happy ... if you do that you will find that you are probably pretty good.   I have a friend who is a songwriter. She has a voice to die for ... like angels singing.. until 3 years ago she would never sing ... saying things like "I wish I could sing, I am so embarrassed of my voice" when I heard her I kicked her in the pants ..

she is incredible but just didn't know it and fear kept her from entertaining people for all of her life to that point ... now she has no fear but it was the fear that stopped her initially