I think what happens is that once something has been 'invented' it is improved on until there is a 'best shape'. There is always someone who will try to improve on that, or who just alters it for altering sake, but after a while of people wanting the ultra-mod, the tried and tested shape actually proves to be the best.
It is much easier to see the time on a wrist watch with hands if ones's eyesight is poor, or if totally blind the hands can be felt. A car is always happier on four wheels. A camera is better camera shaped because it fits the hands more easily, although there is more than one shape that will do that.
What does change for the better in the main is the technology under the bonnet (hood) of whatever the item is. It doesn't always prove better from the start. The early Sinclair electronic watch went though batteries almost faster than they could be made and needed two hands to tell the time. The original digital cameras gave small blocky images. For the most part the modern derivatives of both are better than the old technology.