Why do people say it's wrong to generalize?

Why do people say it's wrong to generalize people when a good majority of times generalizations are true? Generalizing people beforehand is just a defense mechanism if anything. Singling everyone out as an individual is tiring and often leads people to just become bigots. So why not just generalize then wait for people to show you they're not to be generalized then just go with everyone from there? Everyone is a little bit racist anyway