Um..how much information do you want? I teach a lecture to vision sciences graduate students on pupil dilation and constriction
The drops block the receptors that activate the iris muscles that cause pupil constriction. Since your pupils are unable to constrict, they remain dilated. Because it works by binding to and blocking receptors, you have to wait until it washes out- there's not an easy way to reverse it.
There's probably stuff you could use (choline, acetylcholine) to shorten the effects, but my guess is no one's bothered to do the safety studies, because washing out in an hour or six is thought to be an acceptable side effect (as opposed to unknown side effects of adding a second set of drops to the eyes) .
It's considered to be short acting, so it's possible that the alternatives are worse. They do it because they have to have the pupil fully dilated to see the entire retina and make sure there's nothing horribly wrong with it.