Earlier quoted context omitted.
Let us assume it is not overseas buyers - what factors are left? Is it just access to cheap credit that drives investment into the housing market all over the globe?
Why not? It seems obvious that cheap credit would drive up prices. For example people could just be bidding up to what they can pay off in about 30 years. If they simply do that, cheaper credit would rise house prices.
Maybe another factor is that people who are not first time buyers now have a lot more money from the sale of their previous house. We always focus on the first time buyers, but those are only a small part of the market, and for couples with two high incomes, houses are affordable still. So maybe higher assortative mating leads to higher spending potential in first time buyers, and the rest of the money just comes from people selling their first houses and upgrading. It's quite hard to model though, I don't really see a way to test this hypothesis in a few spare minutes here or there like many simpler theories are.