(Side note: letters from multiple sources isn’t impossible, given that you did that with the letters from the DE member of your race in the PS, and during F&F which was from DE of your race or Almorra if you were vigil )
It was impossible from a budget standpoint. Having to write 5 letters per dungeon meant we’d have to write 40 letters total instead of 8. From a logistical standpoint it would have driven up costs for localization and editing. These mails were generated very late in the development process. Multiplying them would have resulted in the wordcount ballooning by 5 times.