Edit: To clarify, I’m wondering about workers who contribute to Google’s own core competency of creating software, not bus drivers, janitors, etc. for which Google’s benefits would be a larger fraction of their pay.
The Tech Equity Collaborative has a report giving some reasons (https://techequitycollaborative.org/download-the-contract-wo...), but they seem to boil down to big companies are so inefficient at hiring (e.g. “adding full-time, direct employees requires multiple levels of sign-off, budgetary allocations, and often numerous reviews and approvals”) that they are leaving money on the table (“tech companies often pay contracting agencies enough for the agencies to offer family-supporting wages”). I’m not sure I buy that.