Because in almost every case I easily find well over $400 dollars worth of repairs on a house, frequently thousands of dollars of repairs.
If you wanted to get specialists you would have to hire an electrician, a plumber, a window and door specialist, an engineer, a roofer, a HVAC specialist, and a general contractor. Those trades are not trained to systematically go through a home and find problems, they are trained to fix a problem that is shown to them.
So, in addition to having a nightmare of scheduling all those people and the incredible cost as well (sure a roofer will come out for free but try to get an electrician to do that. Hell, just an engineer is over $400). The solution is to hire a home inspector to inform the clients whether they need specialists. The clients then can choose or not choose to have further evaluations by those specialists.
If I miss something I compensate the clients. If I ever missed anything huge I am insured for $100,000 dollars in E&O (errors and omissions) insurance. So yeah, there is a warranty on my inspections and reports.
I do the inspection, walk the clients through and answer all their questions. After they know the condition of the home I ask for payment. I have never had a client indicate that I should not get the full amount and I have always gotten paid. That proves to me my services are valued.