I think the key with both was that there was a "plausible" explanation for why the bubble valuations made sense.
In the 2000s, you had the "New Economy" bullshit: The internet was changing everything and the most important thing was to start a company that carved out your niche of the New Everything. Profits or even cashflow fundamentals could wait until after the land-grab.
There was a kernel of truth of course, the Internet has changed a great deal. That's what made the argument sound plausible.
With the housing bubble, the argument was "real-estate never goes down," "the population keeps getting larger so demand always goes up." "houses are real things with real value."
Again there's a kernel of truth to the argument --- just enough to hide the reality of the bubble from people who don't take a hard look at the numbers with a critical eye.
And moreover, during 2000 as well as the housing bubble, very many people who participated believed there was a bubble. Goldman Sachs were long on housing derivatives for years before they went short in the final few months.
There's a lot of money to be made running with the herd in a bubble so long as you veer off before the herd goes off the cliff.