Because there are so many moving parts to it, and probably stems from theater days, where local troupes could do the same play, and let audiences determine which they preferred. And the theater experience was far more interactive with the audiences, and could see a wide variety set design, performance and costuming. Each performance was quite unique but, until we had film, couldn't be fully replicate each performance. So, it is far more acceptable to have several different performances. And that seems to carry over within the film world.