Because they contain that raw sound that newer bands just can't maintain, sustain or in some cases even create.
But that's just false. Can you honestly say that the Truckers, Wu, and the Peacemakers don't have a raw sound (especially live). Hell Patterson Hood (lead singer for DBT) is known for collapsing on stage in a fit of bad ass musical passion. Sure bands for middle schoolers are slick but when weren't they?