it's natural. people always want to have bigger or better toys. it's an innate desire to improve.
although spending more on the paintgun does not neccesarily make you a better player, there's a sense of pride & confidence in using a good marker or gun you've just upgraded. on the field, the less you worry about your paintgun, the better your concentration is on the game itself.
personally, i find customizing my paintgun almost as fun as actually playing with it. after a while, the paintgun becomes more than just a tool. it's a thing a beauty, a masterpiece.
kinda like cars. now we don't need sports cars. why are they there? because they symbolize the love of people for cars & the epitome of engineering.
same is true for paintguns. some players just want to perfect it.
but why complain? generally, money into the marker manufacturing industry means better markers in terms of performance, durability, efficiency, not to mention more beautiful. also cheaper. law of supply & demand. higher demand, lower prices. there's also better sport promotion, etc.