I think it's mostly because I get started thinking about suitability of plot points, both in regards to position in story and in regards to each other. So, if you wanted to have an escalating arc of tension, controlling for that is nontrivial. If you want to write something that's generally noir, you'll want to break out plot events by genre. And which plot events really build on each other is difficult to determine programmatically. Even if you had general "act one complications," "act two complications", "act three complications", each list showing an escalation in average impact... well, not all plots have a three-act structure. (And not all plots escalate like that.)

Obviously, in a just-for-fun setting, this all matters less; playing with constraints, apparent contradictions, and counterintuitive things stretches the creative muscles and can be a lot of fun. And you do see that sort of fun, quirky randomness in things like the card up above. But when I start thinking about things systematically – and I do see plots and overall story structure as a systematic thing – I want to see more interaction between the individual elements.

It would be pretty easy to set up lists in the demographics generator which pulled setting, character, and situation values and stuck them into the various plot forms – you can pretty much set that up in the generators as they are now. But for a more sophisticated plot arc generator, there's a lot of meta thinking that I'm not sure how I'd model in programming terms.