Others explained where the titles come from. Your post got me thinking about what one compares it to. An avid diver (which many of us are, even a one or two trip/year intermediate rec. diver like me) might compare the title 'master' against seasoned dive guides with thousands of dives, top quality instructors, advanced tech. divers who cave dive without stirring up silt, people with a GUE Fundamentals tec. pass, people with extensive experience diving in more demanding environments, etc...
But what kind of 'crowd' do most divers fairly early in their diving compare themselves against? Once you get OW, Nitrox and AOW, and you wonder what's next or whether you're done with training and cert.s, who do you compare yourself to?
Maybe a local dive shop or club group that offers courses and organized dive trips to ocean diving in benign conditions? If so, you might dive amongst people with OW, AOW and maybe Nitrox cert.s, and some with DM, Assistant Instructor or Instructor cert.s.
Master Scuba Diver is a recreational cert. If we ignore the dive professionals, by the time you qualify for the Master Scuba Diver certification, you probably have developed a substantial level of mastery in basic OW diving beyond what you had when you finished the OW course, and even your AOW, and you have broader experience.
So if 'mastery' is designed relative to where you started, it's not a bad term. If it's considered in the context of recreational diving and in your local benign conditions vacation diver group, it may not be pretentious or overreaching.
Some of us judge it harshly because we compare it to a context many people don't have or don't use.
To use your analogy, not all black belts are created equal. I imagine the harder core trained black belts might sometimes roll their eyes at some of the others who have them.