This might sound like a complex or intricate statement, but it is deceptively simple.
If you graph housing price over time, is the graph increasing or decreasing? If it is increasing then housing is a good investment and therefore becoming less affordable, if it's decreasing then housing is not a good investment because it is becoming cheaper and therefore more affordable.
It is supply and demand. As long as supply is restricted homes will increase in value, and if homes increase in value, that's a pretty good indicator supply is not increasing.
This is in aggregate, of course.
Homeowners want their homes to appreciate in value, and many, particularly old people, are using homes as their main vehicles of investment, so of course any policy to increase supply is bad for them. This creates an incredibly powerful political force.
How do you fight that political force?