Because if you're buying a house for a house, then it (mostly, generally¹) doesn't matter that the housing market has moved downwards and you'll be underwater until you pay down a bit on the house. You still have a house! So long as you can afford the mortgage — something you should have already planned for given you bought the house — you can still afford it. You might have remorse at having paid +$100k right before the market moved, but that's what the parent is saying: that's buyer's remorse, that is speculating on the price, when instead, you should be saying "am I willing (and financially able) to trade $400k for a home to live in?"

(¹I don't want to get too much into the sidetrack that is "but if you're underwater you could lose your home" — yes… that's possible. The example here is a 20% fall in prices — which would be astounding to those of us wanting a home, the thing of dreams — but you put 20% in the down payment, so a single mortgage payment & you're no longer underwater. In reality, the price drop (in rent, but let's work with what we got) was 3.7%. (Don't get me wrong, I'd take that too, as a renter.))