Because I can, and when I do it is serving a purpose to me. Whether it makes sense to someone else is not relevant since what I am doing is for my personal use only. Our society is full of examples of personalizing to one's own use. In the case of movies/TV shows this hasn't always been...
I've actually done this many times over the years for my kids. It is not difficult to do. I use software I've purchased for various aspects of it but there may be some free-ware ones as well. Just PM me if needed. People can say what they want to about the integrity of the original film, but...