Part of the reason that Consultants are paid more than employees is, as mentioned, the TOTAL compensation package.
A nice benefits package runs around 50-60% of base pay. Consultants are more or less expected to pick up all those "benefits" themselves, and they don't get them at the individual rate. And a lot of time consultants only work 9-10 months a year.
At the end of the year, if the consultant is paying for medical insurance, group term insurance, into his own retirement plans, etc, etc., his final money in pocket is probably not all that far away from what a well paid professional in his field is making.
That said, a heck of a lot of consultants/contractors kind of don't put maybe as much into retirements as maybe they should. Talk to guys who were in the ME/Design/Drafting fields when CAD came in, suddenly the market for contract board guys who didn't "do CAD" dried up quickly and a lot of guys were hung out to dry