If you buy a stock, even on margin and the price falls and you sell it, you lose your money immediately. If, OTOH, you don't sell your stock and ride it out, probably you only have a paper loss.
The same things is reversed, IF you can't pay your mortgage and default. You lose your home AND you lose whatever equity you have in it; however, since you don't have any equity with an all interest mortgage, you haven't lost what the value of the house used to be or has become. You didn't have any money in it.
If you own your house and don't have to move, you don't "lose" anything at all, when/if prices go down. Neither do you GAIN anything, when/if prices go up......until you sell your house.
Regardless of whether you have money in it or not, you are going to be held accountable by your creditors. Hence if you buy a house and it's value declines you have lost money whether you have equity or not.