How do I configure Apache to use UPnP?

In home networks based on modern routers supporting UPnP, torrent clients just work: they ask the UPnP gateway to open a port for them when needed. Yet, every guide I have seen about setting up a web server includes steps which manually enable port forwarding on the router. Why aren't web servers using UPnP for this purpose? Is there a technical limitation preventing them from doing so? To make the question less open-ended, how do I configure Apache to use UPnP?