Time and fuel to pick up said parts.
Various insurances and trade body registrations, and family cars aren't free to run are they so there's still running costs.
Some will take the pee but there's plenty of costs people don't think about they just see the headline figure and then think they're paying themselves £100ph.
Lets be honest mate youre never more than 10-15miles away from any trade supplier, I'm minutes away from about 5. A few miles in a diesel van...pence.
It does make me laugh these various bodies and registrations they have to be a part of, do they actually have to go back in and be assessed regularly? Like for instance a Mortgage Advisor does every 6-12months?
My example was a labourer, I must admit, that's the first line and they wont have any of these outgoings as theyre the labourers after all. But for some of these traders theyre on £300-500 per day. I drive my car to work, with car insurance etc etc. So I still have expenses myself and don't get anywhere near half that. In on an average british wage.
But like you say some, not all are taking the pee. Id say half are taking the pee, the remaining 50% half of them are sub standard. The market has been diluted in the past 5-10 years I feel. Makes it harder for the honest ones.