Why can the liver lose most of its function before you notice?

It has no pain receptors in its tissue. The capsule around it hurts if stretched, which is why an inflamed liver can ache, but the organ itself gives no signal as it is destroyed - so cirrhosis frequently reaches an advanced stage before anything obvious happens.

It also has enormous spare capacity. The liver can lose a substantial share of working tissue and still perform every one of its several hundred jobs adequately, because it is built with redundancy and because the remaining cells increase their output. That is the same property that makes living-donor transplants possible, and it means the first clear symptoms often appear only when the reserve is gone.

When it does announce itself, it does so all at once and unmistakably. Jaundice as bilirubin accumulates; fluid in the abdomen as protein production fails; bruising and bleeding as clotting factors stop being made; and confusion as toxins the liver would have processed reach the brain. Each of those is a separate function failing, which is a fair demonstration of how many jobs the organ was quietly doing.