Performance of developers within the same team and between companies differs hugely. Someone with some years of experience may be considered senior, and list some technologies in their resume that they've actually worked with, but their level of mastery may not be what you hope for. The differences you get by probing this or not during interviews will add up across a team, and be a real competitive advantage or disadvantage for your organization. And that goes for culture (hiring assholes, or even something more benign, like hiring people that are inarticulate) as much as it goes for technical skills.

And I don't think people typically lie on a resume. I do think there's a category of people that makes things look nicer than they are. I do think there's another category of people that just have a different perception of "mastery" than I do.