We manage software development as if it were just like manufacturing – by tracking resource utilization. The problem is that software development is more like an act of creation than manufacturing. If you want to go fast (ie, see the benefits from the software as soon as possible) then manage the time, not the resource utilization.
Software development is everything from the concept through the code, and then delivery. Manufacturing is about making 10,000 things all alike. If code were as easy as repeating what we did yesterday, we programmers would copy and paste the code and go home by 9am. From the concept to the code, there is more new material on each iteration that repeated material. As such, manufacturing principals (such as optimizing resource utilization) don’t really make a lot of sense.
Jim Trott discusses it more in the podcast Three Things You Gotta Know (about Lean Agile)