The reason why governments force construction companies to do this is because if they don't, it causes ghettoisation, through precisely the mechanism you descri

The reason why governments force construction companies to do this is because if they don't, it causes ghettoisation, through precisely the mechanism you describe. As a society, we have experimented sufficiently with ghettos as a city design strategy, and determined that they are terrible in all ways. We do not need to repeat that experiment. You don't solve the problem of high housing costs by getting rid of the poor people (if you are in the US, you may read that as "black people": that's what this policy has always turned out to mean in that country). If you want to reduce the cost of living, build more houses.