I don't think low volume is an issue. A lot of these vape shops are buying flavor buy the liter or gallon not a small amount like your typically diyer. I think the main reason is just just the cost/time to do it. Alcohol and PG have been the main carriers for flavoring for a very long time and a lot of their flavor recipes are based on that. Switching to VG I'm sure there would be a some things that have to be changed due to the thickness of VG.
Another thing is that a lot of places say 100% VG when it is in fact only 90% or something like that. They don't account for the flavoring added, which is a little misleading.