Because there ARE good reasons to ask job candidates about the skills they put on their resumes. Some people add the keywords to be buzzword-compliant. Others include a tool or technology in passing, as a summary of 5 years of work with it... and if it's important in your shop, it'e reasonable to ask about it. Find out whether "experienced with ToolName" means "I once started up the app" or "I can make it dance."

My resume barely mentions OS/2 anymore, because I'm unlikely to get a job that requires the knowledge. But if I happened to interview at a company where it mattered (perhaps for legacy tech) or where the interviewer shared an interest ("oh! something we can chat about!") then it's a reasonable topic of discussion.

Also, interviewees expect those questions. They can be a kind of throat clearing. That is... get 'em out of the way.