I'm so very sorry for your loss and I understand exactly how you feel. One of my very first rescues, a young cockatiel, had a congenital liver disease with virtually the same results as you've described upon necropsy. As frustrating as it is, sometimes we really don't know why we lose them.
It might help you better understand the liver (largest internal organ) by comparing it to the skin, the largest organ overall. There are hundreds of reasons we might see a rash, or lesion, redness, other discoloration and what appears to be unexplained injury on our skin.
It would be too easy to see an area of redness and think bug bite, but it might be ringworm, allergy, psoriasis or even cancer, among so many other things.
When a liver looks abnormal it's equally perplexing as to what is behind it without being able to run a series of live bird tests and trial and error treatments
The labs showed a normal result just like the overall function of skin with disease or infection would probably do the same. All this means is that the liver, an incredibly resilient organ, kept working. Right up until it didn't.
It's also quite deceptive in producing within normal limits results at any given time even while diseased.
Finally, it's possible that the labwork wasn't specific or in depth enough to catch the nuances or remote possibilities
A BUN isn't an accurate evaluation of liver function in birds, this is a kidney related test; instead, the vets should be doing Xrays to look for enlargement and studies looking for elevated bile acid.
SOOT, AP and LDH elevations are also likely to be found with liver disease; however SGOT and LDH (cholesterol levels) need to be evaluated by an experienced avian clinician since they are not liver specific.
So there's a lot of "should have's" to look back on, but none of this brings your companion back. This is what I've learned first hand.
And what matters is that you did everything you should have. Consider honoring your bird's life and contribution to you by perhaps providing a good, loving, supportive home for a rescue? The entire avian rescue & rehab org I founded after losing that one little bird is done in honor of the brief time he shared his life with me.