In addition to the others reasons meantioned by others: NIMBYism, political projects, and suboptimal planning. Toronto has a whole subway line going through low density neighborhoods and it’s not like any densification plans are happening there soon. On the other hand Liberty Village was given to hi-rise developers because it was a wedge of industrial land squished between railway and a highway. Now it is a bustling neighborhood but pretty problematic to get in it out with the overloaded streetcar line bearing the brunt.

It is not only about upfront costs, ongoing maintenance is important as well, and one needs certain level of density for the infrastructure to be a bit closer to self-sustainable