This has been happening for a while. Most starter homes in the US are townhomes, detached townhomes or small single family homes in a denser neighborhood. Through the years, the building code has changed bit by bit to make those homes unaffordable. It’s similar to how you can pay half the price for a car in Mexico; there are much less mandated safety features. In houses, there are new energy codes (good for the environment) additional safety features like fire sprinklers and other similar things. Additionally, labor is more expensive, appliances and building materials are more expansive.
On the other side, you have people who have lived in their house for decades. The house (actually land) value has increased steadily and maybe they’ve kept it up, remodeling or putting in an addition. Now their kids are all moved out, they’ve retired and they’re ready to downsize, but the house they bought so long ago has appreciated and selling it to downsize would trigger a huge tax event on the appreciated value. They’re better off (financially) to keep it, pushing new buyers to look elsewhere.
It’s a complex problem intermixed with policy and also all the corporations mentioned elsewhere who have learned to profit from the broken system.