It's because it would mean looking at immigration numbers. If immigration creates far more demand than there is supply, as it does in various first world countries, then it makes house prices shoot up. But this is not a popular topic, as voters with houses don't care about their house prices going up, and voters without houses are generally younger and don't have the capacity to hear economic statements on immigration without auto-converting them into racial statements they've been warned about on CNN.
Depends on the place, but e.g. in the UK it's massively high. Almost as high as the US. (Just talking legal immigration; I don't know how to interpret illegal immigration stats.)
The UK takes in about 700k people a year net, and builds about 30k houses a year (which is probably about enough to cope with "natural" population growth.
Supply and demand. Dwellings get smaller (or subdivided) because people will pay more and get less. Landlords appear to solve the problem (and make a profit from the fact) that young people can't raise the capital to buy a full house themselves any more.
It's not the only factor - the jump to two-income households also raised prices massively and transferred a load of wealth to existing home owners back when that happened - but I'd say it's by far the largest factor today.
