The short answer is, "because your brain thinks it's being poisoned." Bet you didn't expect that, huh? Vomit happens for one of three reasons: something irritates your brain, something irritates your gut, or something throws your hormones out of whack (usually pregnancy). When your eyes see something different from what the fluid in your inner ears is telling your brain (i.e. when you're catapulting through space on a carnival ride) it's irritating. Your brain suspects foul play, and just to be sure, ejects the contents of your stomach.