While at-will seems like firing quickly is common, it's really not. In a large company there are often (not always) many HR hoops to go through in order to fire. Often these have to do with legal processes. For example, an employee might have developed alcoholism. A company cannot legal dismiss the employee if the employee is willing to go to rehab. They can fire a contractor/consultant (provided the contract allows for that).

It also has to do with morale. If Jim just disappears and he's a FTE, that makes people nervous. They might start looking at a different positions. If Sally the consultant is gone tomorrow, oh well. There is often an "us" v "them" relationship between FTEs and consultants. It's not necessarily antagonistic, but it's not as amicable as FTE to FTE.