Why do internal organs of the body not get stronger , the same way muscles do after injury
Some do, though it depends on what you mean by “strength”.
For example, if you have a kidney removed, the remaining kidney will enlarge and increase function (going from 50% function to around 75%). Similarly, organs like the liver will regrow if part of it is removed.
If you’re asking why organs don’t increase capacity and function if required, they absolutely do. To a limit. When you drink alcohol, the liver increases its capacity to deal with it. This can demonstrated both with blood tests, imaging and also by the body’s ability to accommodate to the effects.
However, if levels exceed capacity in the short term it can’t keep up, an alternative metabolism pathway is used and less pleasant compounds are produced as a by-product, making worse hangovers.
If exposure is high for long periods, the need for increased capacity results in high cell turnover, which can result in scar tissue and excess growth (cirrhosis) and potentially cancers.
That isn’t an example of lack of adaptation. It’s evidence that of an attempt to keep up but demands constantly exceeding reasonable capacity.
Physical strength is a property of skeletal muscles because they move and exert forces. Other organs don’t do that, so “strength” is not even a property they possess in the first place. It simply makes no sense to describe the “strength” of a pancreas or spleen or thyroid.