Why? Because it's there, and because the commercial ones may not always be there. If I have to order and pay shipping from a place 500 miles away to get something that is made from ingredients I can buy at the country store, I'll learn how to make it myself. If the same concoction that works on film also works on prints, so much the better.
When I wrote the article "Non-chromogenic Antiscorbutic Developers for Black and White", which can still be viewed on www.unblinkingeye.com, there was one component, metol, that I could not get at small town grocery or hardware stores. P-aminophenol would have served, but I did not think of getting it from acetaminophen.