The amount of information available for the role is skewed in favour of the interviewer. They know the full scope of the job, the salary budgeted for the role, how well the company has done, how desperate they are to hire, how short staffed they are, how good /bad the boss is, work expectations and obligations etc etc etc....
You as an interviewee know none of the above.
Your current salary is the only thing they don't know.
Even in terms of salary expectations. They might as well ask "can you take a wild guess as to what we might pay someone in this position with your skill set."
These are businesses not charities, if your salary expectation was lower than what they budgeted, do you really think they would say?
Even giving a salary range, all the employer hears is the low end.
My typical response to the current salary question is "my salary is competitive but let's focus on the new position. What's the salary range which has been budgeted? "
In only 12 years of working, I've managed to increase my salary from 14k-20k-26k-32k-42k-48k-75k and I could never have made such jumps answering the salary question.