Why are cars at Carmax so overpriced?

I'm 17, and my dad got me a car from carmax back in March. He got me a 1999 Jeep Cherokee (4.0 Liter, 2 door), with about 52,000 miles, for about $10k. Don't get me wrong, I'm very grateful that my dad got me a car, but it seems like $10k is highly overpriced for a 1999 Cherokee. I looked up the Kelley Blue Book (KBB) price for it, and it wasn't even $4k (that was considering it to be in excellent condition). I know that carmax claims to make sure the car is in good, working order, but $6k more than KBB? I know KBB prices are a little low, but still. Why are carmax prices so high?