Think of the number of extra user interface panels and gizmos and views and pop-ups and menus that users would have to wrangle in order to build, edit, query, visualize, and maintain 3D multiple story mixed use buildings in a city simulation, instead of having a simple 1:1 correspondence between space on the 2D map and zone type.
The closest thing SimCity comes to mixed use zoning is multi-layered Arcologies, and the interface to those is pretty complicated and unwieldy. Imagine if every building and skyscraper in the city were that complex!
Sim Tower supports mixed use zoning (with different discrete rooms in the same building), but just within one high rise tower, not a whole city. It's as much a game about pedestrian traffic, elevators, queuing and congestion, as an economic simulation.
The Sims is whole a game unto itself at a different level of abstraction, that lets you build multi-story residential and commercial buildings with walls, doors, windows, furniture, etc. But it doesn't simulate "urban dynamics" between zones, just "personal dynamics" between the people and the architecture and contents of the buildings. The Sims 3 was scaled up so you could seamlessly walk around small towns, but The Sims 4 went back to focusing on houses or apartments in individual lots or flats, instead of expanding to simulating entire cities.
It would be a very unpopular game, but there are people who would love it.
Or you can change the zoning into japanese zoning, where the UI can stay simple as you escalate the permission level of a zone. https://code.uberinternal.com/T2597239
I think the problem - in SimCity, Cities: Skylines, other video games, and real life - is that if you're building for aesthetics, there are many right answers, but if you're building for efficiency, there's only one right answer. The reason so many mid-size American cities have converged upon 1/4 acre lots with a road hierarchy, 3-lane boulevards, commercial strip malls on the boulevards, office parks, and freeways to link suburbs is that that's the most efficient way to satisfy the constraints of 2-3 child nuclear families who want privacy and a patch of greenery that's all their own and have varied destinations that each support too little traffic to be worth building mass transit to. The reason many megacities (I'm thinking particularly of NYC + Asian megacities like Beijing, Taipei, Hong Kong, Shanghai, etc.) have converged upon high-rise condominiums with street-level retail taking up whole city blocks with abundant bus & mass transit routes is because at a certain population density the suburban model ceases to work, and this becomes the only feasible way to move people to work & shopping. Interestingly, both models show up in Cities: Skylines, and the transition point between them is usually a painful inflection point in your city's development.
The quarter acre of manicured grass, the per-working adult car, the maximum occupancy limits, the minimum square footage requirements, zoning in general all work to keep the "riff raff" out economically.
Nothing about sprawl housing is efficient. Its almost maximally inefficient - you get the worst of everything, the traffic and congestion problems of density without any of the actual density because the people centralize on an urban core and surround it in extraordinarily wide expanses of cloned stick houses. But that inefficiency is baked in by design to push out those who cannot afford to participate in such inefficiency. You don't need gates on your community if the area is unlivable for the working poor for miles around.
I grew up in suburban New England. That's inefficiency - triangular acre lots, empty space with no road access, single-lane roads, three lefts that get you back to your starting location, driving to the next town (or New Hampshire, often) for the nearest commercial zone, the "Massachusetts left" because there's no protected left-turn lane at most intersections, major roads that terminate at a stop sign on a freeway where people routinely drive 85 mph. The suburban sprawl of Silicon Valley was a breath of fresh air by comparison - the lights on the 3-lane boulevards here are at least timed in rational ways with protected turn lanes.
Hell, they have traffic lights here - I grew up 2 houses down from the intersection of 3 state highways at a stop sign, and we fought for about a decade, fruitlessly, to get a traffic light and crosswalk there. It backs up for literally a mile in each direction during rush hour, and accidents are common.
This one efficient model you are talking about is merely the solution to one set of tradeoff decisions in your area, with other tradeoffs being chosen in other places.
https://www.citylab.com/transportation/2012/04/invention-jay...
Certain features were hard-coded specifically for surface street intersections. So you couldn't simulate a Kowloon Walled City level of density, unless you count the arcologies. The car assumption is as built in to the game as it is to real cities.
Power was supplied exclusively by perpetual-motion hydropower dams, enabled by the terraforming tools: flatten, raise land, add water, build dams on the falling-water squares.
Anyway, the point is that if city planners these days are simplifying their task anywhere near SimCity levels of abstraction, the ensuing disaster won't be one that can be selected from a pull-down menu.
https://en.wikipedia.org/wiki/General_Motors_streetcar_consp...
[0]https://www.youtube.com/watch?v=z75lu0wKbX8
Someone else posted the "General Motors streetcar conspiracy" already.