Esham sampled alot of black sabbath... there's one off stories that counter the overall trend... but...
Metal just simply doesn't have enough rhythm/soul to make a good beat.. the parts that do contain the right rhythm/melody elements are so few and far between that you wouldn't even know it was metal
Plus, its easy to just find some junky guitar player to replay riffs, rather than sample again.. very few musicians that can't be duplicated in metal.. unlike jazz/early r&b which was focused primarily on amazing musicians creating new rhythms...