Because it is cool
Because when people feel lonely , imagine ponies or other friends cherring you up is a good way . really.
Honestly, I am not this kind of people - ask my good friend napoleon bonaparte if you don't believe: isn't that true , napoleon?- because I am not as cool as they are, but that don't mean I don't have social issues, and having imaginary friend doesn't mean you have to be the shy guy that spent all the time in internet.
They love ponies that are 20% cooler than badasses that will judge you for being different.so they want to interact with them, the good thing that they have the imagination to do that. And doing that is cool make them feel better. Some of them like to share their lifestlye, great ! and Many of the pictures are touching. no offence , but moar pinkie, plz . Their way to act are different with you? Great! but plz don't treat them as aliens- personally I don't have problem with aliens, but......oh, I am off topic!.
I don't know what is the great mess with autistic people, Honestly I was austistic as a child, and I know that they are the people that need more love than anyone. They are austistic ? maybe. They really need love and friendship, words that aren't in the dictionary of the bullies that will mock them only because they have more imagination seems funny. So they choose ponies to lead them out of this misery.
What is the problem with that? Anyway, they are great !