I always was under the inprsssion that it's due to the fact that heat degrades chlorophyll at a rapid pace. At 212f it's hot enough to degrade the chlorophyll, and hot enough to have sped up some conversion from THCa to THC, but remains well below the boiling point of THC. If it gets hot enough (not really possible with water at normal pressure) THC will certainly boil off (vaporize). That's essentially how I understand fractional distillation to work. They have precision control over boiling points and vacuum and heat to a certain temperature and then capture that vapor in a separate container. A certain range of temp results in a certain "fraction" of product (wanted or unwanted), and there is certainly some overlap.
I could be wrong, but I think boiling the leaves will result in the loss of some THC ( and the majority of chlorophyll. )
Interesting read: