Tata/Infosys etc are some of the largest single employers of H-1Bs, but collectively it really does seem that the majority of new H-1Bs every year are in fact issued to people already in the country on F-1 student visas.

Go ahead and ask any immigrant who had a H-1B about their path. I am confident you will come to the same conclusion.

I am 100% certain you have a selection bias. Imagine if you worked in Tata/Infosys and started asking around.

From my experience most h1s are imported by vendors. I've seen a lot of really small "consulting companies" who does this. I had heard over and over again that most regular companies are terrified of having to deal with H1s, due to extra cost and lots of uncertainty.

Selection bias or not - I am saying across any employers out there, go ahead and ask folks. Maybe you don't realize how many people in your companies are or were on H-1B and came via this route.

The H-1B program is heavily used by graduates from US institutions.

I would welcome someone actually studying this rigorously. I am confident my observations will hold. There is also no evidence to assume that Indian consulting services occupy the majority of H-1Bs.