Because legally then it's a copy and not a format conversion.
This story has a lot of details left out and is a magnet for illinformed commentators: the reason the originals are destroyed is because it is legal to do so for the purposes of format conversion, but not legal to retain the originals under copyright law.
This story has a lot of details left out and is a magnet for illinformed commentators: the reason the originals are destroyed is because it is legal to do so for the purposes of format conversion, but not legal to retain the originals under copyright law.