BJJ guys cross train in Judo to up their standup grappling game and vise versa, it has less to do with lack of skill and everything to do with knowledgeable fighters who have a realistic view of their system knowing where they could improve, and going to a specialist for what they what. I'm sure their are plenty who believe FMA empty hands is good enough that they could walk into a ring and take a championship title. Same as their are purists in every art who view what they do as superior to anything and everything else. I've known kung fu guys who claim they would destroy an MMA fighter if only they could use their deadliest techniques and I've known Kung Fu guys who go and train San Da and even Muay Thai because they realize the value of specialized training. In the 90s there were countless tae kwon do practitioners who knew they didn't need anything else to fight, now you have TKD schools offering a myriad of other training options along side their art.