Big picture: Nice-sounding economic theories claim that stock market valuations are rational, but those theories are mostly bullshit. As soon as the actual humans in the real-world stock market get excited, or scared, or otherwise emotional, they mostly stop caring about all that stupid boring gotta-do-math "rational" stuff.

Yes, eventually, the humans have to sober up, and stock market valuations return to approximately what the economic theories say they should be. But the dangers of betting on that "eventually" are very well known: https://quoteinvestigator.com/2011/08/09/remain-solvent/