It's part of the rules that make the game challenging.

[There's no harm done in the end really; Oikawa just trots after the ball to pick it up and bounce it a few times, before starting to walk to the edge of the court.]

If there's no challenge, it's not quite as fun, you know? Part of why anyone plays any kinds of games with rules, right? [Smiling, then backing up a bit to toss the ball up - he might just be doing one of his killer serves.]