To note, 187k people per sq. km is not actually unrealistic - there are neighborhoods in the world with this level of density such as Ayal Nasir in Dubai, and even some with this level of density and the same order of magnitude in population such as the Imbaba neighborhood of Cairo in Egypt. But this is a detail and mostly besides the point.
Put it that way - in SimCity as long as crime is down and some other factors are adequate, they won't leave even if they are miserable. But if crime increases, then they leave, even if they are in good health and make a nice income nonetheless.
Doesn't seem that unreasonable, actually. High-income people leave crime-ridden areas, happens approximately everywhere
Also, do you claim to know exactly how and why crime happens?
I'd say the population limit stems from the game's map size limit. The largest SimCity 3000 map is 32 square kilometers, and Magnasanti packs 6 million residents in there. That's 187,000 people per square kilometer.
For comparison, Manhattan is one of the densest areas in the US, and it has 28,000 residents per square kilometer. Manila, the densest city in the world, has 41,000 per square km.
I am unsure I would describe a city seven times denser than Manhattan as "reasonable", at least in a game designed to replicate cities as they currently exist in the real world, and I am also unsure I would describe the decision to limit the map size in a video game as "ideological".