Originally Posted by GuitarBizarre

Well as far as the academy are concerned, anything that isn't avant-garde, jazz, or classical, is 'popular music'. Rock music and pop music being not that far apart, and metal being essentially rock music, its not surprising they all follow the same basic structures a lot.

What is perhaps surprising is that very little popular music is through composed, (Composed with few or no returns to an identifiable section) at least, not outside of those forms predisposed to building and repetition as a musical device, like downtempo or ambient.

Edit: Also, to further address the formula comment above, most of those works were composed using forms because they were for formal occasions where listeners had very strict expectations of the format. When modernism, Wagner, Cage, and even Chopin in the romantic period, come around, the idea of rigid obedience to an expected structure, or even to an audience, was considered outmoded. After all, romanticism was often concerned with the idea of god working through man, rather than critical acclaim or similar. Modernism largely concerned itself with abandoning structure entirely, hence the early works from Schoenberg's atonal period like Pierrot Lunaire, which was entirely atonal and observed no structure, rythmic, melodic, or otherwise.