Probably didn't fix the head gaskets because he's trying to get the OP back on the road as quickly as possible for the least amount possible.
Besides, he said they'd be good for another 30,000 miles. The "prediction" is probably based on experience. If you see a lot of unmolested 6.0's come in with head gasket issues with around 110,000 miles on them, you know that the engine has a tendency to do that. Maybe it will, maybe it won't, but you don't want to be left on the side of the road when some preventative maintenance will keep that from happening for that particular reason.