CAPA is an acronym for “Corrective Action / Preventive Action”. If you've been involved with the Quality profession for longer than 10 minutes, you've probably heard this term.
The short answer is because CAPA makes a general assumption that is often incorrect.
To explain this, let's quickly cover the definitions for these two terms. For sake of consistency, I'll use the definitions provided in ISO 9000:2015, “Quality management systems–Fundamentals and Vocabulary”.
3.21.1
Preventive Action
action to eliminate the cause of a potential nonconformity or other potential undesirable situation
Note 1 to entry: There can be more than one cause for a potential nonconformity.
Note 2 to entry: Preventive action is taken to prevent occurrence whereas corrective action is taken to prevent recurrence.
3.12.2
corrective action
action to eliminate the cause of a nonconformity (3.6.9) and to prevent recurrence
Note 1 to entry: There can be more than one cause for a nonconformity.
Note 2 to entry: Corrective action is taken to prevent recurrence whereas preventive action (3.12.1) is taken to prevent occurrence.
Note 3 to entry: This constitutes one of the common terms and core definitions for ISO management system standards given in Annex SL of the Consolidated ISO Supplement to the ISO/IEC Directives, Part 1. The original definition has been modified by adding Notes 1 and 2 to entry.
It is vitally important to note that each definition begins with “action to eliminate the cause”. This is the “general assumption” mentioned above.
However, there is a MAJOR flaw in the CAPA concept… as it establishes an unrealistic expectation that every problem has an assignable root cause that can be eliminated.
To suggest (or imply) that all defects are the result of an “assignable cause” is to dismiss all that we’ve learned from Walter A. Shewhart & W. Edwards Deming!
Rather than repeat an earlier article, read: Corrective Action "Whac-A-Mole".
Amazingly, ISO 13485:2016, “Medical devices — Quality management systems — Requirements for regulatory purposes” was re-affirmed in 2020 (with no changes). This standard retains the CAPA concept (addressed in ISO 13485, sec. 8.5) under the misguided belief that it will one day achieve the mythical “Zero Defects”… completely oblivious that it is perpetuating the inefficiency of Corrective Action "Whac-A-Mole".
And this delusion continues to be reinforced by the US FDA (Food & Drug Administration) promoting the CAPA concept: https://www.fda.gov/corrective-and-preventive-actions-capa
Sadly, the medical device & pharmaceutical industries continue to doggedly embrace the ignorance promoted by the cult of “Zero Defects”.