Utilize your financial knowledge, give your answer or opinion to the input question or subject . Answer format is not limited.

Why do some people say a house “not an investment”?
I invested in single family homes and made ok.  Houses can be an investment. (though the OP seems to equate "house" with primary residence)   Just like any other investment buying houses has risks.   I would not treat your primary residence or a vacation  home as an investment.  That is asking for trouble, but for many many years it was safe to assume that you would make a good return on it, and many people did.   If you evaluate the numbers for purchase price, rental market, etc and find that rentals or flipping is worth your exposure then by all means, do it.  But treating your primary residence as an investment apparently is what that comment means. Just like the stock market, many people have gotten wealthy on homes and there are lots of people who lost their shirts.