I've been personally surprised that this doesn't come up more often. Most mature codebases span a decade or more, and especially in the web, there's so many layers of evolving technologies that were introduced during this time, resulting in complex geological layers forming in the codebase as the patterns evolved. That is not only a lot of lines of code, but a lot of nuance as well. Even a basic problem like routing can make these AIs fall flat on their face because of the sheer context and complexities involved