Hm, yes I'm most familiar with the London D&D Meetup, and my own Meetup I ran for a few years. Lots of GMs & lots of players, but number of GMs always the limiting factor. Certainly weaker GMs can struggle to keep a steady group in that environment.I suspect that's because you haven't been exposed to environments where there's a sharply limited amount of GMs, and as such, the ones present can get away with things that wouldn't fly where its easier to shop for a better GM.
Edit: Do you think bad/untrustworthy GMs are more common than bad/untrustworthy players? While I won't play with most GMs, I've always taken the view that's because my standards are unreasonable, not that most GMs are objectively terrible.