Because brains are split into sections called lobes. Some lobes deal with motor function, some with sensory function, some with communication, some with speech and language, and some with intellectual function. There are crossovers between them.
Many, many children have developmental disabilities that don't have a physical origin. Take DD1. It took until she was 2 years 9 months for anyone to listen to me. Until then I was told that I was neurotic, that I was 'reading the text books' and 'being a nurse'. She was falling over for no reason. I was told she had inner ear infections, that she was wearing sandals which caused unsteadiness.
I couldn't control her. I had to barricade her room with two stair gates stacked on top of each other, then I had to add a plank of wood at the bottom to stop her sliding out from underneath it. I had to use reins at all times. I had to get an electromagnetic lock for the front door and mount the switch high up in the architrave so that she couldn't co-ordinate pressing the button and pulling the handle down, because she had learned to climb over the stair gates on the porch door and escape towards the main road.
When she got to preschool, they scoffed at my concern that the door to a ramp was open, and the ramp ran alongside the building, so she was likely to walk off the edge of it. When I returned from her taster session, the door was shut and they said 'I see what you mean'.
She was quickly assessed as needing 1:1 support in preschool and the LA funded that immediately. But she only got seen at the hospital when she had witnessed falls at the preschool that couldn't have been trips or slips. Bear in mind that I had been taking her to the GP, telling them this for almost a year.
Even when seen at the hospital, they weren't going to give her any scans. However, they did do an EEG which showed epilepsy. That EEG showed uneven discharges in the brain, so a scan was recommended. The CT scan came back 'normal' and the paediatrician smugly wrote 'as expected'. However, he also ordered an MRI, which showed her brain malformation.
My point is that DD1 was 'able bodied' (although developmentally delayed and wobbly). If it was not for her epilepsy, we would never have got a scan and we would never have known that she has a brain malformation.
DD1's geneticist is convinced that she has a genetic syndrome. Genome mapping was in its infancy when she was young and she was entered into the Deciphering Developmental Delay genome mapping project. They couldn't find it. Then she was entered into the 100,000 genomes project. They still can't find it. The Baylor Institute wrote to the geneticist, asking for her records a couple of years ago. They had found a handful of genetic codes that were similar and wanted to compare the symptoms. I don't know if that will go anywhere.
Ultimately, DD1 is hugely disabled. She has no independence. She has just 10 minutes ago come to me to confess that she 'poured too much cereal' - she has massive impulse control difficulties - at the age of 20. DDs 2&3 get frustrated that she 'gets away with' certain behaviour, but the reality is that if I told her off for everything she did wrong, I'd be telling her off all day.
My point, if you get to the end of this, is that being able to walk, being able bodied, is absolutely no marker of ability to function. DD1 does use a wheelchair and is physically disabled now. But it is by far the least disabling aspect of her SN.