For the same reason lots of us have more than 1 vape. There are subtleties to every piece of glass, even extremely similar pieces can have quite a different feel to them. A smaller piece may give better flavor with less diffusion, a bigger piece may give smoother hits with more diffusion, sometimes you want no drag, sometimes you might want a bit more drag. And as you said, even the purely scientific (no fancy shapes or colors) stuff has a definite art factor to them. Just as there's no vape thats perfect for every situation, IMO its the same for glass. There may be a single piece that will fill most situations, but there's always gonna be another piece that can fulfill one particular niche perfectly.