Pretty much any genre nowadays has its "cookie cutter" bands. The bands that don't necessarily stand out, with maybe the exception of a catchy song or two, but they have all the "elements" of their own particular style. A lot of that boils down to production. Many of the modern rock and metal bands sound similar in that they have a standard for what a rock or metal band should sound like. And music is more and more production-based everyday rather than musical. And when you think about it, it's pretty linear - everything is set to a tempo and matched to a grid, same key, not much room for pitch fluxuation (mainly vocals), among other things. Music production TECHNOLOGY has come a long way, but music production itself is kind of pingeon-holing itself into these "standards" for how genres are supposed to sound. And that comes down to the personal taste of the producer. Of course, people want to compare to the radio standard and get noticed and make money too.
Another thing is the business aspect. Record labels are suffering from piracy and seem to push out as many products as they can and as quick as they can. So the market is a bit overcrowded.