I think that becasue the parts aren`t so common on the used market, the same form of research goes into any upgrades that may be made, mostly leaving only cheap stock parts for said used market, and nobody wants to buy those for the most part. Again, the guy just doodling around doesn`t care, and the die-hard knows exactly what he wants and needs before he buys it. Come to think of it, I don`t think I`ve ever seen someone "casually" buy a seven string or modify it, which you know pretty well is par for the course for many 6 string players...
And it `s a vicious cycle, less upgrades means less used parts, means less peoply trying to DIY them on the cheap, leading to a smaller demand for parts overall, leading to the mabufacturers believeing there isn`t a market for 7 string parts, leading to a lower percieved amount of available upgrades, leading to less "random" upgrades and more "surgical" ones, leading to less upgrades which still means less used parts... Just a theory.
