Investors are not blindly giving these companies cash (insert tired Theranos joke here). They see far more information we do, and they do more due diligence than most investors do in publicly traded companies, including (and especially) during the first dot-com bubble. That ended rather catastrophically, and Silicon Valley was never in danger of being "destroyed for good."
Fundamentally, the underlying inspiration for these huge infusions is debt. On the microeconomic scale, it will take a long time for Airbnb to generate $3.5bn in profit, that has already been put into it. On the macro scale, tens of trillions of US debt make zero interest rates possible, which in turn creates a lot of fake money looking for returns on investment.
Fundamentally, the underlying inspiration is the growth of these companies. The fact that interest rates are so low, and that investors are chasing yield elsewhere, helps to enable it. However, these deals would still happen even if debt were more expensive. On the other hand, if the company weren't showing promising growth, then the deals would not happen.
