Because developers don't keep track of possible state changes that follows a callback? Mobile apps are not like C code with an entrypoint, you can't like dump all virtual memory to disk and write back to resume. It's all callbacks and object instantiations, and listing all objects is already a best-effort task.

It won't be impossible to architect an app so that there would be an instance of a class that store all relevant states and load on creation, of course, and I'm guessing that level of competence is mid-six-figure quality in app world.