My DD has used a wheelchair since she was 2 and 1/2, so she is very experienced and we are experienced pushing her. There is something about a wheelchair that makes you invisible to other people. That is not something that new users are aware of, but experienced users are used to it.
I think, as someone else mentioned, people don't see things too well at their "butt" level. It doesn't really occur to a lot of people that the wheelchair user is there because it is below their eye level. (Sort of like motorcycles on the road, people don't see them either because they are not what people are used to seeing).
Since my DD has been using a wheelchair as long as she can remember, her "mental picture" of how much space she takes up includes all the different parts of her wheelchair. She knows
exactly how much space her wheelchair takes up because it is part of her. She doesn't usually run into things (or people), although people stop short in front of her, run in front of her from the side and she had had people nearly nearly land in her lap because they are not watching where they are going while doing things like trying to frame a picture.
A lot of people
assume that
ecvs or power wheelchairs have brakes and can stop quickly if they run in front. For the record, ecvs and wheelchairs don't have any brakes; they stop because they are no longer being given power to go forward. They take longer to stop the faster they are going and when going down a hill. A manual (push) wheelchair is hard to stop on a hill (gravity makes it go faster down the hill and it's hard to stop).
A lot of the people who use wheelchairs and ecvs at WDW are not experienced at using them so they don't some of these things about stopping them. They also may not realize how much space they take up in the wheelchair. Their "mental picture" of themselves is the amount of space they normally take up and they are not used to their feet extending out 18-24 inches more than the rest of their body. So, they miscalculate their position sometimes.
When you add all of those things together (people walking who are not really seeing wheelchair/ecv users and people in wheelchairs/ecvs who are not aware of how much space they are taking up), you are going to have some accidents.
And a lot of the walkers and drivers are not familiar with the parks and don't know where they are going. Some are just excited and not really looking where they are going for that reason.
So, I don't think it's a wheelchair or scooter problem, it's a people (of all kinds) problem.