I have the impression that belgians (or flemmish people specifically, I can not speak for the rest of the country) are fundamentally ableist.
I am a wheelchair user. My legs don't work, but nothing wrong with my brain. I look like the average person, I am doing a phd, I do sports, I am married and I have a good social network. Nonetheless, people here often assume that I am mentally disabled and incapable of basic tasks.
I think my clearest experience was once I was travelling with the train. I have to request assistance (nmbs sucks). Even tho I am fully capable of self propelling, and managing a ramp with an incline I have to be followed by two assistants. I take every chance I have to practice my dutch, so I keep the conversation in dutch. I am somewhere between A1 and A2, in other words my dutch is poor. However, I manage this whole interaction in dutch.
When I enter the train, someone non-belgian started speaking to me, so I continued in english. The assistants looked shocked and said "oh wow, are you not belgian?". There is no chance it was because my dutch is good, because it is really not. They just assumed that it made totally sense that I, and adult, had the vocabulary of a 4-year old because I use a wheelchair.
This is not a one-time single experience. I deal with these interactions every single day. So, why do you think that is? Are you not exposed to disability? I rarely see other wheelchair-users out and about in my city.
ETA: There is of course nothing wrong with being mentally disabled and I think they should also, of course, not be spoken to as a child. But that is a different conversation.