The point is that if I send you a gif, you currently don't need to tell whatever program you opened it in that it's meant to loop. When I send you a gif, I expect it to be looped on your end. It's part of the gif package that we've come to love.
Of course the loop flag doesn't strictly need to exist, but if someone has to perform an extra step to do what gifs were already doing, then no one will use it as a replacement.