Answer:
Because lip reading is HARD
Explanation:
You can't be 100% accurate lip reader. Since someone is deaf they cant hear, so all they can go off of is your lips. Don't you think it would be easier to just look at someones hands. Also I think that a deaf person would prefer to use sign languages because other people around them might be deaf too. But maybe they would talk if someone they were talking to did not know sign language.