We do, because it is not just about there being land there. It is what replaces what it is currently being used for.
The UK has good growing and grazing land. Once you build on it that is lost. You have increased population now being fed by less land area, which means ever more dependence upon food imports - in a world suffering from climate change impacts. It's simply not smart policy.
The South East in particular is vastly overcrowded. Less so elsewhere in the country. And again that is simply because standard market forces pull people to a particular location. We essentially have a policy of pulling people to where the businesses are - crowding cities and clogging up roads, rather than forcing businesses to go where the people are.
And we know where the end game is, because we've done it dozens of times before. The squalor of 19th century Manchester, Birmingham and Leeds is legendary. Yet we seem to have forgotten.
There is too much pandering to business going on, and not enough stick metered out to them because they are failing to spread themselves around the place sufficiently.