Why? Would it not be in their self-interest? Enlightened self-interest, that is. If it is, and they still have no interest, what makes them choose otherwise? How do we free them from that kind of conditioning?
It especially isn’t going to work as soon as you reach the scale where tribalism sets in. That’s a natural human behaviour and cannot be eliminated.
That is true but also overstated. Over here in Europe we're tribalist AF going down to the village level, doesn't mean that we're at each other's throats. At least off the football pitch, that is.
The only way to prevent that tendency from dominating society is with the structure imposed by a government.
You're overstating the power that governments have -- they all, by necessity, even the likes of North Korea, govern by assent or acquiescence from the governed it's a simple numbers game. It is a question of culture, not of having police at every corner. Who, btw, in many places do the exact opposite of reducing tribal tensions.
Like, how exactly would you envision anarchism working in NYC, with the current population of NYC?
NYC isn't a good place to start moving towards an anarchist municipality. Plenty of anarchists in NYC, doing their neigbourhood thing, but capturing Manhattan is pretty much impossible without full smaller cities haven gotten the bug first. It's like starting a D&D campaign as low-level character and saying "but this is pointless, I can't even slay ancient dragons".
Attempting the impossible is a sure-fire way to be disappointed. To feel disheartened, powerless, fatalist. To then fail to achieve the possible. Consider Anarchism not as a vision that is to be realised, or even provable in your lifetime, but as a compass to guide your direction: Can you take a step? Then what's stopping you? Let things yet beyond the horizon be things beyond the horizon, they might not even exist any more once you get there. What's the worst that can happen, that you made the world a bit of a better place? I'd take that risk.