If you've got a company which is worth a billion dollars in assets but is in too unfashionable an industry to be trading at more than half that in stock (and there isn't a more obvious problem you've missed like unsustainable debt), someone will eventually buy it to liquidate those assets, and earn the holders a nice little profit on the way. And value investing is all about the other side of the beauty contest - looking for the companies investors don't think are handsome which are strong enough to end up generating the sort of profits that will eventually lead investors to conclude they're not that ugly after all.

The irrationality is more on the upside: tech stocks whose fundamentals make little sense you suspect will go up in value in the short term anyway because of FOMO. A lot of people have made fortunes on those kind of bets, and there are definitely people day trading WSB hyped stocks who don't forget to sell.