You have not.
Let me try explaining it a different way. Imagine, for a moment, that you’re someone who isn’t very bright. Like, not severely so, but you struggle with a certain complex tasks that your peers seem to be able to grasp with ease. Imagine that, for all your life, you’ve been mocked for this. In school, other kids called you names and did cartoonish impressions of you. Now, imagine you hear about this game called D&D. It sounds fun. You give it a try. While playing this game, one of the other players at the table plays a character with an 8 intelligence (slightly below average). They portray this character as practically incapable of counting past 10. They talk in a voice that sounds a lot like the voices your bullies used to use when doing mocking impressions of you. Everyone else at the table laughs at his antics. How do you think this would make you feel? Do you think you would want to keep playing this game?
Now imagine instead that you’re someone who just straight-up doesn’t like people with mental disabilities. You know that isn’t a very popular opinion, so you mostly keep it to yourself. But behind closed doors, you just don’t like them. Now imagine you also play D&D. And one of your friends who plays it with you has a character with 8 intelligence, and plays him like I described in the previous paragraph. How would that make you feel? Would you want to keep playing D&D? Would you perhaps be more willing to risk letting a joke at the expense of the mentally disabled slip from time to time with that group?
Now think about what kind of environment such portrayals might foster in the long-term.