As someone who has around 3500 man hours invested in building a code base, I can say that it takes something to mod for Warband. If you aren't cutting and pasting someone else's code, it actually takes a good deal of time to understand where to find each layer you were looking for. If you think its really easy and just laziness, I suggest you take the same source (1257 was made open source, and thus was the reason another mod could take it and run with it).
As for why each author doesn't make their mod open source as well, it probably has to do with the 1000+ man hours needed for building a full conversion mod, even if as most modders do the base was someone else's project (with however many unsolved problems it had when it was abandoned/made OSP). At 3500+ hours, users can bite me if they think I'll make the last lines I wrote OSP, but something that was done early or took trivial effort (around 3-5 full man weeks) has been made OSP in the Rigale forum. But, its hardly the same as the mods I spent a year or more on: Perisno 0.7 series, Warsword Conquest, or Phantasy Calradia's successor "Phantasy 2018". Now, the OP could say "none of those are finished and they all suck", to which I'd say, none of those was even written 100% by me, but the other authors made huge strides both before and after and each has something different. So, having some perspective, I'd say your little adventure in micro-modding is expecting pretty huge results for pretty tiny time investment. If you're saying, well, why isn't Mount &Blade Warband script more like a AAA studio, I'd reply the time needed to put a mod out with Warband script and the community provided models, script examples, and 10+ years know how is actually quite reasonable. I expect you'd burn far more than 1000-3000 hours doing the same mod in another platform, counting art and other tools that you'd need to build from scratch. You could of course justify switching to another engine purely for purposes of getting money back from consumers; Taleworlds expects mods to be free, and pretty much this kills some modders from being able to pour a few thousand man hours into something that can never, ever, pay them back for the lost time and wages. But that's a different topic, isn't it? Let's assume no one lives such a bourgeois life that they actually need to pay for groceries (for example, mom and dad furnish a roof, groceries, clothes, and hot water for annual baths) - the best things in life are free right? Except your time, since you sound like you take yourself seriously, although you seriously don't have much of a resume of finished mods to lean on. (this the OP, not the other commentators). I could be wrong - I usually am. But at least I put the hours in. If the OP thinks its so banal to mod, go mod. We're waiting to try it out. Hell, anyone can do it. But you have to be serious first. Are you, in fact, serious?