It's money. Money, money, money.
Go look at the margin that Express is earning with a traditional employee model. Then look at the margin Ground earns with a contractor model. The margin on those Ground packages can't and won't be maintained after a shift to an employee model. The Ground business model is what makes the margin so high, and if Ground wasn't so horribly managed and inefficient it would be higher.
You need to look at average. Combining divisions could save enough to make up for the 'extra' expense of ground drivers being employees.
First off, you ignore the fact that ground drivers are ALREADY employees, just paid through so-called 'independent' businesses. You seem to think that Fedex would need to pay more then the ISPs do. One thing the ISP model has done is show FEDEX how cheaply some people will work. And with the fantastic profits you ISPs claim, there is NO reason other than the unions that fedex hasn't already dropped the ISP model. Fedex could take those great profits and even pay drivers a little more. Fedex doesn't need multiple managers at each location.
Secondly, eliminating the ISP would free-up that 'profit' they take and add it to fedex's bottom line instead. The ISP is useless EXCEPT for making it harder to unionize. You seem to think that having a multitude of ISPs is somehow more efficient than one employer having the economy of numbers. That's just limited thinking.
Fedex can do maintenance, bookkeeping, vehicle purchase and maintenance ALL cheaper than thousands of separate individuals can get those services/products for. Fedex inherited the contractor model, and has tried to see if it will work for them. I don't think that the ISP model gives them the ability to make changes quickly and the model has the inherent problems of small employers getting quality workers with incentive to remain employed.