Ask HN: Why are start ups today raising so much money?

Amazon, which I would assume is one of the most capital intensive tech companies in the last 20 years, raised $54 million in its IPO. I am sure they have acquired other forms of financing since, but private start ups (Uber, Pinterest, Spotify) today are raising $1B+ in private equity rounds. Why do these start ups today require so much additional capital (in equity) compared to those of the mid 90s? What are these companies doing that is so capital intensive?