Primarily because the companies listed succeeded in capitalizing on Software Eating the World (Thiel).

Software companies grew the way they did because, for a while, there was little regulatory capture, with tech doing what tech wanted, at least with the unicorns. Thiel, PG have commented on this.

There was a recent article on HN from O'Reilly - the new wave of breakthroughs anticipated, in biotech, might not have room for engineer CEOs, nor the unicorn growth of software-based start-ups.