I used to breeze through whiteboard interviews; software was a hobby, problem solving a joy. But as I got older and came to identify with being a software developer (we are NOT engineers, more below) I had a harder and harder time with them. Interviewed by some junior twit half my age and a quarter my experience, sniffing and fidgeting and making misleading suggestions, I would freeze up and couldn't come up with solutions. As I walked back to my car I effortlessly solved the problems in my head.
A whiteboard interview is like doing fundamental research in fluid dynamics in open ocean. What it's really testing is your ability to concentrate under scrutiny, you may as well be asked to design an operating system while being strip-searched in public.
I only freelance now and I won't do code in an interview at all, since my potential employer could be anywhere in the world.
Software development is farther from engineering than economics is from science. Engineers have standards and discipline, software developers have "personal coding styles" that are as difficult for anyone else to read as they can make them, and their code is poorly structured. Engineering is vastly more disciplined and not subject to fads like agile and scrum.