25 years ago, I sold my previous house. It recently sold for 4x what I sold it for, a 300% ROI. Was selling it a financial mistake?

Consider that I didn't pay property taxes, insurance, maintenance, and squatter eviction costs for the last 25 years.

Consider that the S&P500 went up 561%.

Buying a house is not a good wealth builder strategy.

somehow, I never learned to use a spreadsheet

Can’t argue with the math but one thing about a mortgage is it takes no effort. Just pay the bill and time does the rest. With stock investment there’s an emotional attachment to every major swing, should you buy or should you sell is never far from your thoughts. Using a home as an investment may not make the most return but it’s easy as long as you can pay the bill.

Real estate prices swing around, wax and wane, just like the stock market does. It's just that there is no real estate price ticker.

Investing in the S&P 500 is even less effort. The last time I bought a house, I had to carefully read and sign about 50 pages of legal papers. Buying stocks is just pushing a button.

In places like the Bay Area, property values pretty much never go down in price, and the YoY relative increase is on par with that of index funds. Combine that with rental income, and you can very likely beat an index fund. Mortgages offer much more leverage than margin loans, which helps offset the upfront capital cost.

For decades, property values kept going up in Detroit until they didn't.

The value of real estate in Detroit was tied to the auto industry. When it fell, so did real estate prices. The Bay Area's value does come in large part from the tech industry, but it's also a very important port of entry for trans-Pacific immigration and commerce, and that's a lot less likely to change IMO.

But that's beside the point. Substitute Bay Area with Seattle, Los Angeles, Manhattan, or some other area of your choice. Or, better yet, diversify your portfolio by buying real estate in multiple cities. The same options that are available in the stock investment to tune your risk profile are available in real estate investment as well.

not just Detroit…

Huh? I invest tens of thousands a year into stocks and have never once thought should I buy or sell. I just auto buy weekly, it couldn't be easier and I never think about it at all. They even have funds that automatically re-balance as you approach retirement.

Also you are acting like the housing market can't also crash.