They do. Martin Anward got started as a modder, was hired as AI developer for EU4 (did an amazing job, btw), took over that project entirely, and is now the lead for Stellaris. DDRJake was famous for breaking and exploiting systems to the max in EU4, was hired as QA, and now leads that project.
I think the issue is that the number of developers assigned to a project is quite small. Those developers have to code all the new mechanics, features, engine improvements, fix bugs, work on engine optimization… and work on AI - including the core AI functions that they expose for modders to work with and take advantage of. Modders have the luxury of not have to deal with any of that and instead can iterate like crazy with the systems provided. They also don’t have to get changes through the QA process in order to get it out. If a modder breaks something no one really cares.
Of course, there’s also the fact that the modders don’t get paid and donate their time to this, which I’m forever grateful. I wish every strategy game had a dedicated AI programmer that didn’t work on anything but that, but it doesn’t appear to be economical since no one seems to do it, even huge projects with massive budgets like Civ5/6.