It'd be nice if the mtime of the files ikiwiki renders matched the mtime of the source files.
However, this turns out to be more complex than just calling utime() a few times. If a page inlines other, younger pages, then having an older mtime means that an old version of it will be kept in web caches, forcing annoying shift-reloads to see the changed content (for example).
And it's not just inline. The template plugin means that a change to a template can result in changes to how a page gets rendered. The version plugin changes page content without any younger page being involved. And editing one of the html templates and rebuilding the wiki can change every page. All of these need to be reflected in the file mtime to avoid caching problems.