That is what it eventually boils down to is costing money, but if you were capable of doing it you could probably do it yourself with their oversight - a good profile hole will eliminate the need to do a perc test most of the time ie: if the soil looks good enough to the inspector he will let you go ahead - perc tests usually require outside help - testing company as they approve of.
The purpose is to eliminate the possibility that your sewage will not be treated by the soil and made safe for natural recycling. Lets say it hit a crack in the rock underground and ran directly to your well untreated then you made yourself a nice cool glass of lemonade ---
Another thing that can happen is that natural water or seasonal water underground can be close to the surface or seasonally close to the surface. It takes about 6 to 8 feet from this water to be sure that your septc system doesn't polute it and possibly your drinking water.
Unfortunate reality is that inspectors, backhoe operators, well drillers, testers, etc. have to eat and pay outrageous insurance, taxes and labor too, hence the undesireable fees. Thats why it is cheaper if you can DIY.