If you looked over my github, you would see most of my commits are ones that, at work, I wouldn't accept from anyone. The code is sloppy, there are no tests, it looks like someone was just trying to get something working as quickly as possible with no thought for maintainability or understandability. But, of course, that's exactly what I was doing! When I code in my spare time, I'm solving problems that I've run into in my spare time, and I approach it very differently from in my work life. The constraints are different, and so the solutions are different.

So, you can ask to see candidate's existing code if you work at a small enough shop and/or don't care about missing out on candidates who don't have shareable code. But if you are trying to hire at a larger scale, it does not work.

Many great hiring schemes work until a certain scale and then they fall apart. Some hiring schemes also severely tilt your candidate pool towards a certain subset of the population. I'm sure whiteboard style interviewing also has similar issues.