I'm fairly familiar with our carrier's use of outside contractors to haul their freight. I became familiar when sitting in Woodstock, Ontario, and asked why I turned down a fairly cheap load. After I told them we wouldn't move our truck at that rate, they replied that if we didn't, they would have to call an outside carrier to cover it. I offered to help them find one.
While sometimes I get frustrated with our carrier, a certain Puerto Rican reminds me of just how fortunate we are. We are mindful of FedEx' desire to turn a profit, and requirement to return value to their shareholders. Most of the time, they are mindful of our needs to do the same. Driver/contractor turnover is expensive, and I know that FedEx is very aware of that.
Regarding the 2/3 income comment, I wonder how many more miles you had to run to achieve those numbers. I suspect you ran more, at a lower rate than you did with FedEx. Not saying either is better; just very different games.