Density should come hand in hand with power-rebalancing measures, e.g. tax property hikes, more efficient zoning, and government-controlled rent, but practically it doesn't happen.
Land value taxation is one way to reduce speculative land use by landlords, but it's rarely used, unfortunately. And in California it would require overturning Prop 13, a hugely regressive tax policy that has been somehow falsely enshrined as helping the little guy out, when in reality it mostly helps landlords and speculative land hoarders.
The idea that jobs can be placed anywhere flies in the face of pretty much every single city's economic development efforts over the past few decades. It's not realistic, or reasonable.
What can be placed anywhere is housing. The only reason that we don't have higher density is a small number of landowners lobbying to keep everything identical to the way that it was when they moved in, and when they moved in they had just massively changed the landscape from fruit groves to suburban sprawl. They said: change to allow me, but no more. And now they benefit massively by restricting density around them.
The focusing is done due to a whole host of reasons but certainly encouraging jobs to spread out is not a bad idea for a number of reasons - including having options and livability
"Density" in my comment is population density, not construction density. Unfortunately, construction density (and height) is restricted by zoning and all kinds of bylaws. Most residential areas in North America allow only single-family houses and prohibit multi-tenant buildings with 3-4 floors or more. Landlords benefit from such zoning rules immensely, because they drive land price up.