Because:

1. Adults scream, too, we even see so in Monsters University. The screams generate enough power to make a door explode.

2. Adults usually are better at tidying their room, so Monsters don't have to worry about touching their stuff so much.

I do see how it could be that nobody believes children anyway and adults would just put cameras in their room or something and that way the human world would know but in a modern era of photoshop, how would you PROVE the existance of Monsters? Pro-tip: You can't (unless you capture one for scientific research).

But even with all that, we have proof that it does work in Monsters University. If Mike and Sully can extract enough energy to make doors explode in less than an hour, far exceeding the scare record held by Whatsername (let's call her scary lizard woman), why don't they just keep doing that? It might take a little longer pr. door but it would also generate more energy.

Bonus request: The "everything children has touched" rule makes no sense. If children has touched the toys on the floor, surely they've put them on the floor, thereby also having touched the floor (possibly with both hands and feet). How does the rule actually work?