A few possibilities:
1). They're nubs
2) In your case of the timid Hydreigon, perhaps they would be using mixed sets. Some people use mixed sets for an element of surprise, or to make they're offensive Pokemon harder to wall and troll.
3) It could just be that nature is what they were saddled wit. In terms of rare Pokemon (they wouldn't bother going out to find another because it would take to long, so they make due with what they have), or they are just too lazy.