Its not reinventing the wheel, for its attempting to improve on a design to meet a specific purpose or to modify a bullet to fulfill a need due to a shortage, etc.
If we never tried to improve the first invention, i.e. the wheel, we would be riding around on carved stone or wooden wheels cut from a tree trunk or from boards, or at best, old wooded spokes with a steel band. However, we all know modern wheels and tires are far superior to those old designs, and even much better than designs and materials just a few decades or years ago.
Similarly, if we never tried to improve bullets, well, we would all still be shooting carved stones, cast iron, cast lead, round nose jacketed, etc. Just over my lifetime, I have seen tremendous improvements in bullet designs that brought us the monolithics, VLD's, tipped, various controlled and more reliable expansions (rifle and pistol), powder coatings, improved jacket concentrics, etc, etc.
I do understand what you are saying, but today's modern bullets just didn't one day appear. People wanted something for some reason, and often via trial and error, they created something "better".
Just like those old boys who played with old car or plane engines, because they needed or wanted something more than what was available. And just like many of us who were not satisfied with only shooting a couple of hundred yards or so, or being told we could never hit game beyond a certain distance. We challenged the accepted norms, demanded improvements or created them ourselves, and expanded the envelope.