Shut the fuck up retard, favela monkeys have like 10 kids and live in dense as fuck cities.
It's because the buying power has been reduced and the cost of living increased, those who are in the so called "Middle class" have to choose between increasing their competitivity (specialization, postponing reproduction, abortion, etc) or losing their social class.
Density doesn't cut, no matter how cool you wanna look retard, because when people moved from the fields to the city, the density was the same, but the workforce offer was almost exclusively males, this is because the system operated under the familial unit, once that unit was no longer efficient, women entered the workforce progressively each decade, and alongside technological advances, wages plummeted while the costs increased.
Density doesn't fucking cut it, otherwise it wouldn't make sense for poor people to concentrate in cities, exactly it's because capital and opportunities concentrated in places with enough infrastructure, and those who own the capital shilled for the exodus to the cities, at one point the tables turned and the offer surpassed the need, increasing all costs of living while making wages go down.
In this sense i would argue "Density" as you call it, is simply a simptom.