As already said many times, Lab caches are not caches with everything normal caches have so it really does not make sense to count them in some of the statistics requiring the D/T rating, container size, log information, cache location, hidden dates and many others.
Second reason is indeed that they were included much later than the Project-GC was created so some of the stats might not have support for them (yet).
Another reason might be that some cachers really do not count them as caches (since they really are not) and want to show up their stats without them. Everyone has its own point of view how the stats should look like and this is good that Project-GC allows us to customize at least something in the stats.
The last reason can be that it is one of the premium features of PGC in Profile stats, which is one of the things that run the site.
Hope it helped.