Why do people assume I'm dumb because of my stutter?

Why do people assume I'm dumb because of my stutter? I wonder if anyone feels the same, but from my expirience people tend to treat me like I'm dumb or less serious when I stutter. I have had people tell me to "take my time to think about what I want to say", even when I'm well prepared. My stutter doesn't come from me not knowing what I want to say! I have had people treat me like I'm a little child, talking down to me with condescending voice. Idk, maybe I'm just being oversensitive, but I feel like people treat me diffrently on my worse and better stutter days.