Because its a good game. This quote from a classical composer explains why. "Good composers borrow. Great composers steal." What I'm saying is that no great thing is completely original. Nearly all of our iconic works of art have been influenced by other things and have direct references to them. For the composers back then, it was not an offense for another composer to use direct sections from your own works. Jealousy happened, but most of the time, they were flattered that their work was recognized by their fellow composers. Bungie taking the idea of Mass Effect is a compliment, really.