Well, there are a few problems with this.
1. If you pour the cereal in second, there's always the chance that the milk will splash and get everywhere
2. It's hard to get the correct cereal to milk ratio if you pour the milk in first. with milk, you can slowly pour it in until it's just perfect. With cereal, it could all come out at once or take way too long to come out to the amount of milk
3. The cinnamon/sugar in the cereal won't mix with the milk as well (so Cinnamon Toast Crunch wouldn't slap as hard)