Because this is the way it’s done in the real world. Across different countries, different airlines, and even different seasons (winter versus summer) or different routes within the same airline, average passenger weights can be different. There is no universally “correct” average passenger weight, so asking different add-on developers to conform to a specific value is not justified. It’s arguably less realistic to do this actually.
For your application specifically, if you are (or are planning to incorporate) importing a user’s latest SimBrief flight plan using the provided endpoint (documented here), you will note that the average passenger and baggage weight used for that specific flight plan is provided in the <weights> section. So you should not need to hard-code or keep track of what weights a given add-on uses to keep things in sync with SimBrief, just read the values from the OFP data when importing.
Best regards,