See for example, the Sierra club in SF which seems to now be basically a NIMBY group devoted to preventing any new buildings from going up because they might partially block views or cast shadows over existing things.
Some people choose a lifestyle for their families that differs from what might be optimally efficient, for reasons that other's don't prioritize as highly.
Additionally high density housing and cities are vastly more energy efficient than sprawling suburbs based on at least one simple factor: cars.
No one will be building high density housing from recycled timber any time soon.
Large housing projects require less material and construction than low density housing. Why? Because low density housing is essentially doing a relatively giant housing project per person, spread over way more physical distance. People have to live somewhere, so if high density housing isn't made then they have to live in energy and resource wasting low-density housing.
That being said, living in high density will always be more energy efficient as less energy is wasted on locomotion for people, less energy is needed to service garbage pickup/disposal, less electricity is needed for heating/cooling, less material to construct a habitable unit, less energy is wasted transporting people to their work, less energy is wasted getting food to people (one grocery store to serve a whole high density area rather than a large distributed network of grocery stores), less energy wasted on food delivery (delivery driver can just go to one building versus driving the car around the suburbs)... and the list goes on.
New buildings sequester carbon
Carbon sequestration is usually meant to mean carbon emissions are locked up in something, typically CO2 pumped in to a stable geological formation.
Whereas my point is that new buildings are carbon emissions insensive.