[titanomachy]

"Not sure if there are other factors that make centralized waste treatment systems preferable over individual septic systems"

Capacity is the main thing I can think of. If you get dense enough (e.g. 8-story apartment blocks) you'd need a pretty big septic field right next to the housing. Now that's land you can't use for housing or offices. At some point it's more cost-effective to whisk that waste away to some low-value land to do the processing.

I don't know at what level of density that's true, though. It could be pretty high.

[aw1621107]

Yeah, density was the main concern that led me to make my original comment in this thread. I was thinking of a more suburban setting when I wrote the comment you're responding to, but you do have a point (unless local zoning laws allow septic drainage fields to be used for other things, but that I know too little about). If you can't build on top of drainage fields or dual-purpose them, then you'll need to sacrifice some building space, which isn't optimal from a make-the-most-of-a-land-parcel view.