When you don't, you not only risk your life and the lives of others, you risk your equipment. The lives are more important, but the equipment can be costly, and where a track might have some runoff room and a gravel trap to keep you from contact with a wall (and no sharp curbs at all), the street isn't so friendly.
Note: we're not faulting the drifter for their driving here; when you go hard, sometimes things go wrong. But we are faulting them for their choice of venue.
All of that said, check out the video for a rather illustrative example of what can happen on the street even when things don't go horribly wrong.