You can't exactly not work until payment is made, simply because the field is too crowded. So in the starting of a project, you desperately go to a bidding war against the competition where you either compromise on your time or your money or something else of yours. Whether you get paid or not becomes a backseat priority. Offering a discount for early payment is one sort of those things, and let's be honest, most clients value the optionality of not paying at all over paying a discount. Charging an interest on late payments needs to be explicitly mentioned during the bidding process, and that will draw the client away in an already crowded field.