Because breeding. Coupled with the fact they lock ditto until mid-to-late game (sometimes even post-game or rarely never to appear in-game) means if you want to breed starter derps to trade for other starter derps for 'dex completion you need a female to do it. If you just started your game, played on and saved, you're shit out of luck for that, unless you hit the 12% first go or can beg a ditto off somebody for bullshit in return (unlikely during earliest days when people know what they have's desirable because others haven't reached 'em yet). It's not to make it utterly impossible, but drive interaction/trading for the people who don't give as hard a fuck what symbol's on their pokémon, but also want to have the other sets of starters at the same time.
