A key advantage of the assembly line production of these things is fast iterative development. At first, in a 'move fast and break things' way, when doing tests

A key advantage of the assembly line production of these things is fast iterative development. At first, in a 'move fast and break things' way, when doing tests. But just as significantly, by learning about non-fatal flaws in a working design so you can improve it. The 100th rocket you build and attempt to fly will be much more optimized than the 5th. So by ramping production you can develop much quicker. Another related point is tolerance of failure. A failed test for a government program will immediately see pressure to cancel the program by it's political opponents. Any detractors for a company will have a much harder time exerting influence.