Not so long ago, they could go to work, school or social outings and never have to see anyone different from themselves. They could use words like "retard" and "spaz" as insults without thinking of the actual meaning of those words. They could move effortlessly through their world, secure that the infrastructure - both physical and social - was all designed with their needs in mind. The social structures which guided their passage through life were so stable and secure that they never even questioned their existence, the way one never questions a mat's reliability to stay on the floor.
You'd be surprised how hard it is to find a photo that illustrates the idea of a homogeneous population. Finding this took me minutes.
Now, though, things are different.
Now you will see people with all sorts of bodies and all sorts of abilities going about their day, and you'll have to get over the desire to stare, or avert your eyes, and deal with us like actual regular human beings. Now you're expected to think before you speak (or type), and understand that even if you don't feel the term you're using is offensive, someone else might and with good reason.
If you've been brought up with the unspoken belief that one way of being is right, and others are wrong, damaged, broken, shameful, or a source of novelty, then suddenly being expected to turn that whole way of thinking around is a big deal. Change is hard. And that sucks.
You know what else sucks?
Being beaten to a pulp
Being assaulted by someone supposed to care for you
Being left to die in a fire
That's why, when the straight white able cis neurotypical dudebros whine because they've lost their right to use "retard" as an insult or park in the spaces closest to the shopping centre doors, they won't really get a lot of sympathy here.