Because Blizz allows multiple accounts, doesn’t properly take care of hackers (no bans, they just patch hacks every once in awhile), they don’t ban accounts that are boosting (should be easy, you send someone to “hire” the person offering the boosting service, then permanently ban their account when you ask to see proof of their rank) etc etc , every person just about has been to Diamond and Masters and even above.
It means nothing if you meet someone who has been in GM or top 500 because it’s so easily attainable through all of the unethical ways that Blizzard allows, and make no mistake, they allow it. If I was in charge of these things I could come up with endless ideas of how to almost completely get rid of it (you can never completely remove it, but you can come close).
Getting to a high rank, or even above avg (plat) should mean something, it should be special, but it’s definitely not. This can only be resolved through massive changes in their ranking system (especially the matchmaker) but they have been given ample time to resolve it, so at this point I believe they are unable to do so.
What you see is what you get unfortunately.