While your point is valid, we must look at this from a risk point of view. Most agencies are born out of someone's passion to help people and for that person, it's extremely important for them to do great work. There is a greater risk for an agency to invest so many people on one client than it is for that in-house employee to invest his/her time. That's why agencies go above and beyond to do the necessary research to learn as much as they can about that client-- becoming passionate about the comapny's products, mission, and goals.
Although it's true that an the in-house employee knows the daily operations of the company, when it comes to preparation for a social campaign, I have to say that the agency wins again. Unless the in-house person is one of the partners of a company, that job sometimes is just a job, so when that individual leaves the office (weekends, holidays, etc.) they are not as dedicated to responding and/or maintaining the social campaign. Whereas, agencies have a responsibility to see things through from beginning to end.
In addition, there is less pressure to do well for the in-house person because in most cases, the in-house person doesn't necessarily have a supervisor and if he/she does, then the supervisor doesn't even know enough about social media. For an agency, standards are set and the pressure lies in producing unsatisfactory work or worse-- losing the client.
I hope this explains my POV a bit more. Thanks for taking the time to read and comment.