For many people, they will never try to improve their singing because they think they're tone deaf, or just don't have an ear for it. Much of the time, what they really don't have is a capacity for a little punishment...as learning to sing is going to push you through your comfort zone many times over. At the same time, it's well worth it.
The thing is, only about 1 to 2% of the population is actually tone deaf. For them, they can't easily separate sounds into pitch, making it hard for them to sing on key. Even so, it isn't impossible for them...they may never uncover perfect pitch, but can certainly learn relative pitch, which is good enough for karaoke and choir performances.
Have a look at this message thread about singing despite being tone deaf for some more on the matter.