Because they literally don’t have to do anything. The Feat just says they get a 20% discount on non-magical items. They can just go to a town they’ve never been to before, walk into any store they want, and ask the shopkeeper for the employee discount, and according to this feat they just get it. They don’t get advantage on checks to haggle, they don’t have a guild that supplies them materials to craft their own stuff cheaper than market price, they just get an automatic 20% off, no questions asked, carte blanche to tell the DM, “nope, that plate mail only costs me 1,200 gold, it says it right here on my character sheet.”