Going down that path, you may as well use forum descriptions and topic content together with titles to build respective meta descriptions. But that's not the point. I'm not asking for what already exists (
here for example) and I would not even mind phpBB to add dynamic meta tags in core, but rather requesting for what looks a left over to be removed.
Because as it is now, it is blocking the possibility for an extension to set meta keywords/description without altering phpBB, AND the only effect the current setup can have on SEO is, if any (no drama here, it's most likely none) negative (since it's repeated on every pages).