A common reason in my experience is that the things they've built were done very slowly. I've failed my share of interviews because I was having a bad day, certainly, but I have direct professional experience with many people who require hours of dedicated working time to produce even the simplest of pull requests. This isn't a complete justification (if I have professional experience with them doesn't that mean they passed our straightforward coding tests somehow?), but I do think that there's a real selection problem there people are trying to solve.
