Because it's not a real problem, but rather one you've invented, and in the real world we have the exact opposite problem, of huge swathes of desirable residential land all SFZ'd, so you can't even build town homes on a parcel.
It's amazing you keep saying this when I've said multiple times now that I'm living in exactly that kind of city (a large North American city), and it's not the only one with this exact problem. But sure, keep ignoring the data for your simplistic solution.
That's one cause. The other cause that you seem to refuse to understand is creating the wrong kind of density. An urban centre full of 1 bedroom apartments is still going to create urban sprawl because families just can't live there, thus driving them out of the city centre even though they still work there. I just don't get why you refuse to understand this very obvious point.