The car seat phenomenon is real, well studied, and interesting. The explanation is not only logical, but is exactly what you called a "serious" reason: cost. Parents must often buy a new car with the 3rd child, because they cannot fit 3 car seats in their current vehicle.
It says an enormous amount that we simply accept the premise that we must live in or near a major city. We don't have to trap all the livable jobs and housing there, but getting them elsewhere requires bootstrapping with a long lead-in, and we actively attack attempts to do that.
Many highly skilled jobs automatically gravitate towards the population centres because of the pool of the workforce available.
We have a dev team in one of the big cities of New Zealand, we need people, and we interviewed everyone available (yes, this must be on site).
When you have jobs, childcare, education, entertainment, government, etc, all inside cities, people migrate towards cities.
I wish I could move further away from the big city but I'd get paid third of my salary, my SO didn't found any openings, and I'd have to buy a second car so my kid could attend their school.
I see what you're saying but not every job is available outside.
