Why can't you leave a bridle on a horse in the pasture?

The bit on the bridle makes it so when the horse tries to graze, it chokes. Also, the horse can step on the reins and trip. Also, if you leave a bit on a horse unsupervised the horse can severely injure itself....including cutting it's tongue off and/or cutting it's mouth open. You should NEVER tie a horse with a bit in it's mouth or leave a horse unattended with a bit in.