> Then why did houses used to be affordable even in those dense regions with high paying jobs?
Because those city centers have remained the same size while demand for living there continues to increase
More demand for a fixed set of land drives prices up.
Those city centers today are not equivalent to the same city centers 35 year ago.
Just to take it one step further, there are usually geographical reasons why cities are located where they are.
So you also can't just build a new city in central Nebraska and have everyone move there for cheap.
This is besides the entrenchment that happens when industry is in one place for a long time.
> More demand for a fixed set of land drives prices up.
This works because both you and GP specified "[free-standing] house". This is not true of homes, where multiple homes can occupy the same land - just 15 feet higher or lower
Perhaps someday more American cities will discover the third dimension, allowing for cheaper housing