For me, two reasons. One or both of which aren't as rational as I'd like to say they are.
1. When I think of prefit barrels on a custom action, I instantly think of Impact because they're the ones that popularized prefits on custom R700 actions. They have by far the widest selection of prefits and, even though it's not that relevant, by far the most people in the PRS using prefits with their actions.
2. It's still in my brain that Defiance actions just aren't as reliable in adverse conditions compared to other actions. Years ago that was probably more true, especially with their Deviant line of actions which was famous for being unable to handle grit. These days, especially if the anTi/Classic are hunting oriented actions, I assume they've changed their tolerances for better reliability. But those early years really make my brain exclude them from any consideration for basically any purpose. As irrational as it probably is.