You have thousands of instances only to have a /c/games on every single one of them where they'll all have the same GTA6 speculation thread when the GTA6 trailer inevitably drops making the same exact observations and jokes. People will naturally say, "fuck this," and just go to the thread/instance with the most people already, which would almost certainly be the one in lemmy.world.
You make a great point with the shop analogy. But it's not only items being sold, to continue with your analogy, but the culture and vibe of the place as well. Multiple general purpose shops can coexist if they have radically different vibes. One general purpose shop can cater towards Amish people while another can cater towards bodybuilders, and they would have radically different vibes even if the items being sold are almost exactly the same which would nudge you towards going to one shop over another. Maybe you don't like Amish people for being weird about technology. Maybe dudes with bulging muscles weird you out.
Virtually every single Lemmy instance save for like 5 is just trying to be Reddit, basically Reddit minus spez. Seriously, if you blank out the url and everything after the @ for usernames, you wouldn't be able to say, "this is obviously a lemmy.world thread" or "this is such a lemm.ee thread." Outside of those 5, the only distinguishing feature would be if a thread has some chick flashing her titties or some dude getting his dick sucked, you could rule out that thread belonging to SFW instances. That's pretty much it.
I'm not sure what you mean by constitution. Are you talking about a mission statement or statement of values?