This widespread wish to live in new communities is simply a fruit of social conditioning, nothing else.
As for nothing stopping infill, legally and formally, in there isn't anything stopping it from happening, but greenfield development is cheaper, easier and mote lucrative in the short term, for both the city's coffers and the developers, albeit extremely damaging for the city, overall, in the long term. Continue to allow unchecked greenfield development and suburban sprawl is going to break the city's finances, worsen our transit, increase congestion and the need for investment in car infrastructure, etc...
And there's no way back from this if we reach a certain threshold, it just becomes a snowball rolling forever down the hill. The city doesn't need to forbid or make it impossible to develop new neighborhoods, but it could very well start by rebalancing property taxes, so that new suburban developments are not a burden for the entire city. It's ridiculous that people living in central neighborhoods have to subsidize the suburban sprawl with property taxes.