Pensions were a massive burden (just look at the auto companies who still have massive pension liabilities), so it probably made a lot of sense back then.
But now that corporate America has eliminated pensions in favor of 401ks, the contractor/employee juggling just seems wildly inefficient for anybody who's not a janitor.
It feels like now its just a case of the sheep following the herd, for no one reason anymore.
Anybody I talk to about this gives a nebulous response that basically amounts to "Hell if I know, that's just the way everybody does it!"