Why can kidney and liver function be tested by simple blood tests, but other organs cannot? why can't all organs be tested this way? just wondering

Basic biology: The kidney's best-known function is to remove waste from the blood, so that's easy to understand. Liver function tests is actually a misnomer -- these measure stuff released when the liver is injured. Similar tests exist for heart muscle, and blood gases are a marker for lung function, but your history and physical exam are more helpful as screens.