Because to require a particular level of housing near jobs would require a prohibition on offering jobs without developing additional housing, in effect limiting jobs. Even if local governments are given the power to do that, politically, citizens demand that politicians do what they can to encourage job creation, not limit it by some other constraint.
Need for housing ~= {jobs} + {adult students} + {unemployed} + {retired} + {liquidity room} - {All housing in the market}
If the need for housing is positive than actions should be taken to accelerate and promote the development or re-development of new planned accommodations.
If the need is /dire/ (as it is for the large west coast cities) then that the government should take on the buy out and re-development of entire neighborhoods in to areas that are suitable for the continued health and prosperity of the whole (as private interests have clearly failed to do so or regulations have not allowed them to do so).
This planning and redevelopment should also include mass transit and other resources as well as more than meeting 'minimum code' for all construction. (My own views about the inadequacy of such codes are out of the scope of this topic.)
That would help prevent the decline of public transit.
A better question is why haven't cities required sufficient housing near jobs to keep the cost of rent (let alone actual ownership of even a condo) competitive with the suburbs via supply control?