Just Wondering.. Why Do Cops Investigate Car Accidents?

This picture in the LVRJ had me wondering, why police officers are the ones who investigate motor vehicle accidents instead of people in the private sector?
Just in this picture, at a scene of a motorcycle fatal accident in North Las Vegas, shows that least 3 (two motorcycle cops and an officer in the car) but probably many more, will be off the streets while they investigate this accident for several hours, probably around 4 hours and if it is the Nevada Highway Patrol, about 6-8 hours.
Why can't they hire traffic investigators who investigate these accidents and if there is a need for a ticket to be written or an arrest, then call the cops in, or have 1 cop on stand by in major accidents.
Cops spend hours investigating major accidents at the scene and more back at the station, taking them off the street while they cold be doing cop things.  Maybe it could reduce the number of cops and the overtime associated with these bad accidents.
Of course, the union hacks will say the cops are better at it but that is crap, as usual.
And they will say tradition, but cops used to ride horseback and they broke that tradition.
So, just putting it out there.