While everyone else is on the “employers are stupid my boss is dumb ha ha ha” bandwagon let me offer a perfectly logical reason for this:
They’re asking your salary because it’s a better indicator of your skills and employability than anything you tell them.
In jobs with extremely clear, no-doubt-about-it credentials, they don’t ask this question. If Robert Transport’s looking for a truck driver, they have an amount they’re willing to pay and that’s it. They don’t have to ask you what you make because either you’re a truck driver with the appropriate license and endorsements, or you aren’t.
But if you’re the Project Manager, Applied Program Development or some such thing, what does that mean? A title like “Project Manager” could mean you’re a glorified salesperson at a 20-person company. Or it could mean you’re a serious mover and shaker with enormous project management credentials at a huge company. It could mean you’re a junior PM or a senior PM, but of course you aren’t going to tell the interviewer “My title exaggerates my abilities,” are you? Or what if yu’re in “Sales”? “Account executive” could mean someone taking phone calls from customers at $16/hour or someone moving million dollar contracts who makes $16 a minute. Which one do you want to hire to sell $40 million projects? Are you going to go cheap on that one and risk your entire business?
A rather critical way of screening the overblown from the bona fide is what people make. If someone is passing themselves off as a super duper PM with experience handling big projects in a high demand industry and they make peanuts, they’re full of shit. If they make $115,000, well now, you’ve got yourself an ace candidate.