According to Matt Levine's recent column, while you might think that, it wasn't what sunk them in practice. Bidding low in fact worked; it just was inherently limited in scale, which is why they switched to bidding higher. Unfortunately, being wrong in the other direction is very bad.
"I know, I know, the traders are saying: “No, this is stupid, your algorithms will not be 100% precise, some of your ‘lowball’ bids will in fact be too high, and those will be the ones that sellers accept. You’ll get adverse selection and end up losing money.” But that was not Zillow’s actual experience in the first quarter! The actual experience is presumably that some people accidentally got too-high bids, realized they were good and accepted them, but mostly Zillow sent too-low bids to everyone, and some people, for whatever irrational reason — market ignorance or financial necessity or laziness or whatever — accepted the too-low bids. The general point is that there is no reason at all to think that the people on the other side of these trades from Zillow are generally better informed than Zillow is. Sure they know more about their houses than Zillow does, but Zillow knows more about the market, and has more money"
"If you systematically bid too low, you will not do many trades, but you will make a lot of money on each trade. If you systematically bid too high, you will lose money on each trade, and also you will do a whole ton of trades. This is much worse!"
