Why do people think you’re retarded if you have a speech impediment?

Why do people think you’re retarded if you have a speech impediment? No one bullies me anymore since Junior High School but when I stuttered back people use to make fun of it which made me really angry because I couldn’t help it. My stutter has gotten better but I still sometimes get stuck on words or repeat them. I have learned to avoid certain people that might make fun of me because I’m tired of dealing with immature people who have nothing to do but make fun of you so they can feel better about themselves.