The thing is this - configuring a server is one set of problems, building a decent CMS database/backend is another set of problems, and building a decent browser-based UX for content authoring is a third set.
Very few people have all three of these skills, and it's fair to say that 2 and 3 are not yet solved problems. Rolling your own CMS for any non-trivial purpose is always something that sounds like a good idea until you try it, and then you start hitting all of the incredible idiosyncrasies and speed bumps that other CMS have already solved, even if they've done it poorly.
I'm not exactly defending Wordpress and its lousy code, but in my experience with publishing CMSs, if it's powerful enough to flex to non-trivial needs (Modern WP, Drupal, Django, CQ, etc), then it's probably going to feel like a bloated/complex mess to a programmer, because of all the nuances in the problem space.
Having worked on both sides of this one, i'd rather solve the 'scale this crappy software' problem than the 'Build a useable UX solution that does everything we need and works on mobile and in IE8' problem.