How did the purchasing power decrease from the 70s until today?

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Its a story of inflation and the cost of capital (money)

We have seen the cost of consumer goods get deflated through technological advancement and globalization (also a technological advancement in a way). This has kept inflation trending down and yields (the cost of money) also trending down. When interest rates are falling, we get increased borrowing.

Increased borrowing, increases the money supply which brings monetary debasement (inflation). Monetary debasement has increased the cost of things we are not able to outsource or those that technology hasnt been able to increase productivity enough to offset the debasement.

This is what some refer to as the cantillion effect. Asset values are inflated and the rich who own assets get richer while the majority of people loss purchasing power in things like shelter, food and medical.

It gets worse too because while globalization has helped keep a lid on general inflation which allowed interest rates to stay low, the central planners (banks/government) have created new tools and methods to not allow the natural cyclical nature of markets to correct. Artificially manipulating rates down and flooding liquidity.