It's very common for a driver to cause an accident and then leave the scene. We call that a hit and run. So why do they do that? When you get through all the reasons it comes down to them and some fear that something is going to happen to them that they don't want.
One of the most common reasons is that they're drunk. They may not have a driver's license. They many not have insurance on their car. Or there might be a warrant out for their arrest for driving on a suspended license or any number of things.
You have to assume that when a driver hits you and leaves the scene that they are uninsured, whether that driver is found later or not. The bottom line is they are probably not insured. This is why you need to make sure that you get as much information about the accident as you can. Make sure witnesses saw it and are identified so that they can tell what happened and verify that you were in an accident. And, then report it to your insurance company, so that your insurance company can take over to protect you.