Why are horses in fantasy novels so well behaved?

Came up in a discussion on transportation at a con. It really occurred to me that in the vast majority of fantasy novels, horses all behave completely perfectly unless the writer wants to show that a character can't ride well.

Last time I rode a horse that behaved perfectly for the entire ride? I can't remember...

So, tip to writers: Horses act up even if they are being ridden by somebody who knows what they are doing.

They are not machines. They are animals with minds of their own...and characters.

The reason, I suspect, is that most fantasy writers have only ever ridden dude string horses, who tend to be chosen to be lazy and act out only by occasionally refusing to move ;).