Why isn’t he using inflation-adjusted figures for 1971?

Official inflation rates calculation formulas have been adjusted too many times for it to have any value to compare prices over 50 year period.

Why isn’t he comparing the cost per square foot of houses instead,

This is a valid point, it would more precisely account for “hedonistic” adjustment

e average house today is much, much larger than the average house in 1971

is it 5.5x larger?

If the number is 4x instead of 5.5, how would your analysis change on the trend we see here?

Because they are comparing inflation of different things. If it all was done inflation adjusted, income would have gone down, many prices would remain the same and some would go up. It would be less clear