From 1994-1999 I ran ibm.com (whatever that meant). For most of the five years it was just me, one other FTE, and a roving band of 5-10 contractors. Any time I suggested converting the contractors to FTE I got slapped down, even though it eventually meant the loss of a lot of talent.
All I can figure is that the companies want to avoid long term benefits commitments. Healthcare. Pensions or 401ks, etc. They rarely saved money, the burden rate for my team was easily 30-50% more than my burden rate (not that the individuals got that largess, it usually went to their "manager" in the contracting agency).