What you've missed here is that failure points can only be guessed or simulated prior to testing. That analysis is often more expensive than the cost of a test and test article.
So SLS does a LOT of analysis and manages to find and rectify failure points prior to flight. They pay a lot in analysis and inflexible design to do this.
SpaceX does some analysis, but then flies to confirm the analysis early. That way they identify actual failure points and can use sensors to see how close they got to failure.
For instance today I'm sure they learnt a lot from each failed engine, parts on the booster that stopped working, detailed telemetry on ship behavior on flight and sensors for the non-separation of booster from ship.
SpaceX builds to the same high tolerances as other rocket manufacturers, but they don't try to avoid testing through overly rigorous analysis. They also don't gold-plate their manufacturing, instead making tradeoffs to allow cheaper volume production with recovery instead.
So SLS does a LOT of analysis and manages to find and rectify failure points prior to flight. They pay a lot in analysis and inflexible design to do this.
SpaceX does some analysis, but then flies to confirm the analysis early. That way they identify actual failure points and can use sensors to see how close they got to failure.
For instance today I'm sure they learnt a lot from each failed engine, parts on the booster that stopped working, detailed telemetry on ship behavior on flight and sensors for the non-separation of booster from ship.
SpaceX builds to the same high tolerances as other rocket manufacturers, but they don't try to avoid testing through overly rigorous analysis. They also don't gold-plate their manufacturing, instead making tradeoffs to allow cheaper volume production with recovery instead.