Cause SW has so many Matte paintings, half or more of them were mediocre quality at best so Lucas could use them just for a single shot! Remember, Matte paintings weren't exactly cheap or easy to produce, and Lucas chose quantity over quality. Why he doesn't change it is the same as asking why not update the vector graphics used for the death star plans...
Yet despite using probably dozens of different Matte paintings he used the same shots of tie fighters and x-wings over and over and over - like it's too difficuilt to make more!
Not that he should fix anything, he should leave them alone.