There's significantly more information in 2018 or even 2008 to indicate Amazon was worth that valuation. A companies potential has to be adjusted by risk if you want a good valuation, and in 1999 there just wan't enough evidence. This is validated by it dropping dramatically after the bubble popping.

In fact they were only saved because he had secured financing mere months before the collapse, had that not happened Amazon would be listed among the likes of Pets.com.