Why didn’t we do this? Seems cool.

Because it's like 1000x more efficient to move stuff on water vs land, so industrial cities clustered around ports and rivers since it's way easier to move stuff around.

Big cities aren't typically in such an ideal planar geological setup as that. I'm having a hard time imagining how something like that would work in the Bay area, NYC, Seattle, Miami etc

Because it trades most of the benefits of cities for the hassle of suburban life and less efficient food production.

Because of perverse incentives

To be fair, Alexander was writing in the seventies, long after automobiles and the suburb had killed any hope of humane urban planning in the States.