Sampling is an art in it's own right.

Dre is a very talented producer with a long history of success: producing tons of music, bringing gangsta rap to the mainstream, pioneering the gfunk sound, pioneering the business model which have made hip hop labels so damn profitable, assembling super groups of studio musicians to consistently produce very popular music, finding and marketing brand new artists that have skyrocketed on the charts over and over again (eminem?), and more. Dre is an artist, but he's way more than that.

read something: http://en.wikipedia.org/wiki/Dr_dre