Street racing is illegal for a variety of reasons. Most of all, the roads are not designed for this type of use. It exposes other vehicles to a potentially serious accident.
You could find yourself in a street race area without knowing what is happening. This could be horribly dangerous as you may drive into the path of vehicles moving at top speeds.
The biggest risk with street racing is the drivers are going at speeds excessive for the area. These contests usually happen in areas with reduced speed limits. The straight path allows the cars to get up to high speeds. The higher the speed at impact, the more severe an accident will be. It also increases the chances of a deadly crash.
Racers need a straight road to hold a race. These roadways may have intersections, but usually, they have little traffic. The fact that the roads have little traffic is another issue because it means these racers are not expecting any other vehicles to come into the area. They likely will not be watching out for other traffic, which means if you roll into their path, they will not be ready to react quickly to avoid an accident.
Street racing is not legal because it is so dangerous. Even if another vehicle does not come into the area and cause a crash, the racers could wreck their vehicles. Usually, if something happens, the racers will try to leave quickly because not only would they face liability for causing the accident but also for street racing, which could increase any other charges against them. So, if you become a victim in this type of crash, you may have a very difficult time holding anyone responsible.