Monopolies on chemicals and mechanical products are industrial regulations. Changes in those industrial regulations only have to take into account the effects of those regulations on how well the industry serves the public (does it produce enough, is the price right, etc.).
(Hobbyists do exist in those domains, but they don't do mass scale production and patent holders usually don't both with litigation against hobbyists who aren't affecting their profits, so the social harms are minimal.)
Monopolies on software (or on writing books or music) affect the general public as well as an industry. The whole free software movement was (and is) built on people like you and me writing some code - just as the books and music that are available exist because some people who can write/compose well decided to do so.
(Patents also happen to make the software industry perform poorly but the bigger point is that analysis of software patents can't be narrowed to economic analysis.)