If every materia gave the same number then it would be too easy to get the best gear possible. The way it is now keep the economy flowing since you need to tear through tons of gear to get the exact materia you want. I have gone through maybe 100 pieces of gear and I have yet to see a +20 stat. I think when I do get one I'll be very happy. When I'm about to turn gear into materia I get that feeling like when I opened a pack of pokemon cards! I know I'm gonna get something but what will it be and will it be a normal card or a holographic?! I don't want materia to be predictable. That would be boring. Having variety in types and numeric value makes it interesting.