One reason is that I feel it is just a much more harmonious picture.
Or this vs.
I really don't know why I prefer the smaller horses but I do :)
Lets not forget their sense of humor.
Finally, lets face it I'm pretty short lol. It is just so much more comfortable to be able to get the saddle on and off your horse's back, to be able to get the dirt off their but without a step stool and mount without a ladder.
Not that I don't like ladders.....
I have a great time looking around YouTube for people, not kids, riding ponies well and enjoying them the way I do so I will leave you with some of my favorite recent videos.