Github and many other high profile companies have the benefit of a much larger talent funnel than more average companies. They can literally sit back and wait for the proverbial rock-star ninja to make a passionate appeal to come and work for them. They can filter anyone without a 4.0 from a top school (not that they would, but that's not the point) and still be working with a bigger pool than lesser companies do, pre-filtering. Never mind actually selling yourself so those you might want to hire will actually accept your offer.
Lesser companies simply have to take more chances on people if they want to have a shot at growing their teams. They will naturally fail more than companies that don't have to take that shot.
True, they have a bigger talent funnel than smaller companies, but don't forget that they also need far more new hires than smaller companies. I wonder which factor dominates.
Also, GitHub just have 166 employees (https://github.com/about) - presumably not all engineers. Any ten random selected medium sized software companies easily eclipses their volume.
Lesser companies simply have to take more chances on people if they want to have a shot at growing their teams. They will naturally fail more than companies that don't have to take that shot.