There are some companies with so much overhead with regards to hiring people that it is much quicker to just approve a consultant, and the cost/benefit analysis takes this into account. Sometimes it is because project dollars and employee dollars come out of different buckets, with different approval chains. There may be more money in the project budget with a simpler approval process, making it easier to contract than to hire.

Regardless, internally they have a budget for each job you do. It may be simpler for them to simply approve each job as they come, even at a premium, then to fight to get more headcount approved. Theoretically this also gives them the opportunity to cut you on each job, but if you are that awesome they'll keep signing you up.

(And in some dysfunctional companies, headcount is directly related to management authority, so it can be easier politically to spend more on a consultant than to fight to figure out which manager gets to become more important by having another direct report.)