The basic chemistry isn't terribly hard, but producing a consistent product is going to be tricky for a guy in his basement. Theoretically nothing is impossible for a really determined guy in his basement, but in practice if the bar is set high enough you can effectively eliminate the behavior from all but the most extreme people. Ultra-extreme people already get increased scrutiny from law enforcement, so the scope of abuse is at least somewhat containable.
Drug cartels today could manufacture their own guns, yet they don't. Or at least not in mass quantities, given how many guns they purchase from the US through various means (including in some cases directly from the US government).