You forget one very important fact.
Machine downtime.
It takes the better part of one day to re-tool one of the six Mable motor machines to make a different rocket motor. After that is completed there has to be hours of testing to get the settings just right for production of the specific motor. After that motor machine finishes it's production run then things have to stop while the machine is re-tooled to make another motor type. All this can result in 3-4 days of downtime when there is no motor production happening on that machine.
That's lost revenue.
Making motors that have a known track record of strong sales (C6-5, B6-4, A8-3) is the better use of machine time.