Why is a spark plug harder to pass factory tests than you think?

How many tests does a spark plug have to pass?
Semi-finished resistance check, Finished resistance check, Oil-immersion spark test, Ground electrode weld inspection, Center electrode weld inspection, Gap inspection, Appearance inspection, Structural inspection.
Every inspection verifies the product's consistency and reliability.
Even the smallest deviation, if it goes unnoticed, could become a future engine problem.