How is FIP fatal? Why can't we just address the symptoms?

https://en.m.wikipedia.org/wiki/Feline_infectious_peritonitis

As the Wikipedia article says: inflammation in affected tissues (which can lead to organ failure; jaundice is one example), plus the fluid accumulation putting pressure on the lungs and making breathing difficult. A drain could possibly reduce that last effect, but wouldn't help the other organs. Since this is a virus, antibiotics can't do anything but stop secondary infections.

Accepted wisdom is that there is no cure for FIP; treatment is symptomatic and palliative only, i.e. typically the owner is advised to make the cat as comfortable as possible. Prednisone or other immunosuppressive drugs prescribed by a veterinarian may help to prolong the cat's life for a few weeks or months, but may be contraindicated in certain cases due to concomitant infection(s), although this risk can be mitigated somewhat by also treating with antibiotics. Newer approaches using immune modulators are being developed by several companies. In the cited above paper by Legendre and Bartges (2009), three cats who received polyprenyl immunostimulant survived for over a year, and one of those cats whose treatment started in 2006 has been still alive 8 years later as published by Cat Fancy [5]

Effusive ["wet"] FIP usually progresses too rapidly for any meaningful therapy to be attempted.

Wish I had better news for you. Good luck...