Why do soccer/football people think tall people are slow?

I'm really pissed off because I played Keeper in Germany just because I was tall and my coach said I was slow because of it and im not slow at all. Now I live in the US and my new coach won't let me play the other positions not because I'm slow but because I haven't ever played those other positions because my coach in Germany said I'm slow. Don't get me wrong, I love Keeper and im good at it, but I just wanted to experience other positions in my life!