You can define it as under-building, but that's only one side of the political effect. E.g. the UK net immigration rate has been pretty enormous, and it seems lopsided to call it under-building to have not built homes for a giant number of people coming from their previous homes elsewhere in the world to the UK.
Speaking as the son of an immigrant, married to an immigrant, for the people who can only think tribally, and must assume I am doing the same.
Underutilization of land as a result on speculation on the growth of land prices is a major reason, even Manhattan has massive chunks of real estate that are vacant, a land value tax would fix this.
That's an effect of massive demand, not a cause (or not a first-order cause). If you know (and vote for) loads more demand is coming the more you wait, you will wait. If demand were more limited to natural population growth levels, there'd be no point waiting.
I agree that both things are caused by one level or government or another. I'm just saying that under building is not the only explanation. It's probably not even the most natural one. It's maybe a second or effect once you've decided to admit multiple cities' worth of people each year.
Speaking as the son of an immigrant, married to an immigrant, for the people who can only think tribally, and must assume I am doing the same.