Whoever said that put some poor ideas in your head. Don't be too quick to badmouth singing. Voice, more than piano, is typically an accompanying instrument. But unlike just about every other instrument, the tuning is in your head, not on the instrument. It takes more than just belting out sound to coordinate with the rest of the performance. Don't get me wrong, I believe piano is drastically harder, but saying that anyone can just stand up and sing is a bit silly. The difficulty in piano is mainly coordination within your own brain. There isn't very much you do on a daily basis that takes the type of brain coordination that piano does, so it's much more unnatural than singing. (Video games, typing? <-- I don't get a 'backspace' button in piano though...)