Colleges have of tens thousands of people doing a quite limited number of things - mostly classes and teaching, with some support staff or service easily placed

Colleges have of tens thousands of people doing a quite limited number of things - mostly classes and teaching, with some support staff or service easily placed around campus. It's easy to plan for that. I'd guess most campuses in the US can easily be walked across multiple times a day. I went to Purdue, W. Lafayette, about 40K students, and didn't need a car. One could walk between classes and dorms and food trivially. Cities of hundreds of thousands to millions of people have easily thousands and thousands of varying jobs, different size and scale employers, a huge range of skilled and unskilled labor, a vast array of specialists and specialty employers, and even several entire campuses - a massive amount of different requirements. It's not reasonable to expect everyone to have employment within close range, so immediately people need cars to commute. It's not reasonable to put some large employers evenly mixed with housing - no one wants the garbage facility or any of a host of companies next door. Many companies also have huge inflow/outflow requirements like trains, shipping, power, waste, again complicating the picture, since people need to travel larger distances to reach them. Zoning laws didn't just pop up to prevent campus living everywhere - they popped up to solve the issues that occur when living and industry was co-mingled. So while I'd love to have all of life be as serene and pretty as a top university, I also prefer to have specialized jobs and places to do such work, and I don't see how these can possibly be merged.