Because of that user needs to manually install xorg-server.
This is intentional. You could be using xorg-server remotely.
users can install xorg-x11 metapackage. it pulls most of the X tools, some scripts assume to be present. useful for pure X environment without any DE. I recommend setting USE=-fonts for this metapackage though, those old fonts it pulls are pretty obsolete and not really needed.
At the end of install you can print a message like "Now maybe you want to emerge xorg-server", when it is not installed.