Well, "Why don't they just..?" question is not specific to this case. It's a very common question in many other areas of life. In my opinion it's because of general curiosity found in humans. And except mild annoyance it can be a very good way to introspect your position.

For example if others haven't been asking this question we wouldn't got this explanation. And now that I got this explanation of NASA's process I would like to know "Why don't they just make all this process more agile?"

For example why have only one launch in 7 years with expensive hardware? Why not multiple launches with cheaper technology?