Why do we need to substitute an urban apartment for one further away (causing long commutes)?

Is the city "full" or something?

I can't speak for all cities, and clearly Detroit is different from New York, but at least here in Seattle the problem isn't the lack of space but the cost.

Putting aside the reasons (Amazon, Google, Microsoft, etc, all being willing to pay well, people wanting to live in an urban area, etc etc) the ground truth is that developers can charge a ton of money for a residence in Seattle now.

So they do.

No one's building anything affordable because hey - why not get paid a ton more for doing basically the same work by developing a luxury apartment/condo/etc.

So it's not that the city's full, it's that most people can't afford to live here anymore.

The nature of the apartment determining its price is not a very good model. A luxury apartment goes for $1200 in Chicago or $3800 in San Francisco. This is not because the San Francisco apartment has better finishes, but because San Francisco has a tighter supply/demand balance.

The suburbs contribute supply and absorb demand.

If developers built similar quantities of “affordable” units, we would have just as much trouble getting them, but due to waiting lists/lotteries/bribes/whatever instead of rental prices.

Given the sorry state of the apartments left after urban decay, “luxury” apartments are the only kind that won’t be a huge decrease in quality of life to middle class former homeowners.