Fixer is both cheap and abundant, which is great for those of us that enjoy developing our own negatives. However, in the search for local alternatives for film development, it presented a problem. Because it’s so easy to acquire, few have bothered to try making it themselves.
This is not to say that no one has; in fact, there is an active community of folks that buy and mix their own chemicals and achieve a high degree of nuanced control over their results. But ordinarily, they buy from reputable photography suppliers and not from the supermarket.