As someone who spent several years as a solo freelancer/consultant, sometimes offshore, sometimes onshore.

The main reason this type of work is drying up, in my experience, is that there is too much free money flowing around. Why deal with a temporary worker when you can afford to hire them full-time and have them sit in your office?

Furthermore, full-time engineers add a lot more to your valuation than freelancers and consultants do. A client once told me that adding a consultant to their "full-time" force increases the company's valuation by $100,000, but a proper full-time engineer adds $1,000,000 to the valuation.

I'm not sure why, but if I had to guess I'd say it's a combination of a company showing clear signs of growth (more employees == growth), and the lower churn because of 4 year vesting plans.