I agree, who knows why one thing is so scary and others aren't. Ours have actually done pretty good w/ carts. I was prepared for a big training day w/ Gypsy, but she didn't even bat an eye, my brother in law's gelding was the same way. My project pony Genie wasn't to bothered, although we haven't let her get right up close. Haven't yet tested my sister's gelding or my neighbor's mare, truely no idea how they will react, the mare is a real been there done that type, so hopefully she'll be fine, the gelding is so mellow and laid back I 'think' he'll be ok. My dad's horse, I expect to need some training, but we'll see.
Over all we haven't had to many freak about the cart. But I use to ride on a drill team and I remember once being in a parade w/ just us and the chuck wagon people, several of the drill horses were prancing sideways, snorting and really bothered by having the wagons behind us, my horse was more bothered by a plastic bag that came floating down the road.