What I don’t understand is that how is this automatically good for startups?

Say I have a startup that vibe codes “AI for real estate”. What about customer acquisition?

On the other hand, if I’m Zillow, why can’t I just throw a developer on the same feature and automatically have a customer base for it?

If you look at most of the YC funded startups these days, they are just prompt engineers with no go to market strategy and some don’t even have any technical people and are looking for “technical cofounders” that they can underpay with a promise of equity that will statistically be meaningless.