Bubbles are caused by reckless stock purchases leading to insane, unjustifiable valuations, regardless of the company's prospects.

Hysteria + widespread financially illiteracy means there are lots of people prepared to buy AI stocks at any price. They're adamant to get in (FOMO).

Suppose OpenAI will be a successful company into the long term, and you bought its stock at a $5T valuation. After several reporting seasons and subsiding hype, its true worth turns out to be closer $1T. OpenAI could still be enormously successful, but anyone you bought at much over $1T valuation is going to get hurt.