In those decades most consumer goods have become cheaper (adjusted for inflation), but we've seen drastic increases in the cost of housing, medicine, education, and infrastructure. Those are big money pits.

As for why this happened, and continues to happen, and what could be done to fix it... those are hard questions. You can see some good grappling with the subject in this article, and the follow-up listing other people's responses to it:

https://slatestarcodex.com/2017/02/09/considerations-on-cost...

https://slatestarcodex.com/2017/02/17/highlights-from-the-co...