Theses days, IMHO, most of the arrows are shot by post houses trying to protect their turf and ROI.
I suppose that's one way to look at it. Another way is that many, if not most, producers of higher end material (network television series, for instance) have worked with the established post houses for years, don't want to be in the post business themselves (and yes, that includes daily creation, duplication, and distribution), and value the professionalism, understanding of the entire process, and efficiency that the post houses bring to the table, regardless of what they're shooting on. They feel that the post facilities have proven time and time again that they can be trusted to deliver the best product, and deliver it on budget and on a tight turnaround (and yes, that includes dailies), and that the relatively small amount of money that might or might not be saved by doing it themselves is just not worth it when there is already an established network of facilities and an established infrastructure - not to mention newer players as well - that removes all the questions from the table.
Like I said, another way to look at it.