The satellite build costs $200m.

Launch 1 costs $600m: total cost, $800m.

Launch 2 costs $200m: you can build 2 spare satellites for the same cost (assuming you don't pay for the launches that go boom; and even if you do, that still gives you one spare satellite).

Production doesn't work like this all the time, not everyone is the US government.

Building 2 would cost more than double, or it will take twice as long.

There are huge lead times involved, subcontractors with other commitments and other bottlenecks in the pipeline. Not to mention that even if you do manage to build 2 in the same time window at only double the cost you still have the issue with the launch window, a launch accident would most likely ground the provider for a very long time which means you have to find another launch provider.

Spacecom had a launch window from spacex, spacex has a queue of 9-10 more launches already set, there will now be a huge delay and possible cancelation even if Spacecom had another unit they couldn't just launch it next month.

The flip side is that estimating the reliability of a launch vehicle is an inexact science at best given the paucity of actual launches to provide data points (frankly, there was no real way of assessing the reliability of the Falcon 9 other than to note that launch failures or partial failures tend to occur more in the first couple of years of launches of a new rocket type is backed by a reasonable amount of evidence across the history of other launch vehicle programmes)

Given SpaceX's long manifest of scheduled missions, it seems like the people who actually pay for this stuff don't always agree with you. Going with the more expensive option is apparently not a "no-brainer."