Why can't deaf people talk?

They can, it is just that, without the help of hearing aids, they may feel insecure and quite shy. However, with some deaf people who wear hearing aids, you can even tell they are deaf because they talk just like a normal person. People who have been deaf since birth will obviously find it very hard to talk because they have never actually heard what words sound like so can't learn them properly, but if they get hearing aids they will usually be able to hear a lot better and so learn how to talk properly quite quickly.