Go ask one to its face. It'll either be mad that you implied it's less than human, or it'll be mad that you called it a person when it's so obviously superior to people.
Sapient creatures are people, yes. Especially when they speak human/humanoid language, which is true for many (most?) dragons in tabletop settings and even Lord of the Rings. In Faerun (D&D) they can even easily transform into humanoids.
If they keep taking the slow sheep, they'll evolve to be faster and make the whole thing less convenient. Dragons live a long time, so they know from experience that in the long run it will be easier to only eat the fastest sheep.