I have heard a few people say that "a house is not an investment" as an argument to stay in an apartment. (To me, this seems like a stupid reason, but that is not my question.)

Of course, this is not the only place I have seen this argument. I've seen some scammy TV commercials trying to sell financial advice claiming the same thing.

So, the way I see it, I'm building up equity. When I need to, I can use the house as collateral, or I can buy a new house with that equity,, etc.

So, why do people say that that isn't an investment?