Because Geocaching.com says so.
As a general rule cache types that do not have a log book are no longer permitted. Web cam caches, virtuals, There have been exceptions, such as the recent gift of virtual caches to some people, and of course Earth caches. Also a recent decision is that log books are no longer needed for event.
If you think a place is awesome enough for a virtual, then it is awesome enough for an ordinary cache, the advantage of which is that an ordinary cache provides the challenge of the hunt, and the fun of the find. A real cache is also more child friendly, as it may contain treasure.
