Why do we devalue blue collar jobs?

People in the United States tend to look down on blue collar jobs despite the fact that they are necessary. For example if the county,state, city groundskeepers didn't mow the lawn and trim shrubbery the roads would be so overgrown no one would be able to use them. Also if their where no Custodians your place of work and your children's school would be so unsanitary it would be hazardous to your health to be in the buildings. Yet we continue to put people down who do these jobs. We also consider people who provide these services overpaid but think its fine that CEO's who don't provide a hell of alot to the general public deserve to make tens of millions. Why?