It's not just the overhead of showing up, it's the opportunity cost of not doing contiguous hours at a bigger job. It's very difficult to fill up a day with 45-minute jobs all over town, so he's basically working part-time if he takes a small job.
Where I live , there is a "call out fee". that prevents tradesmen from not being paid for time costs driving to job investigations .... The customer also pays for the needed diagnosis time too. So I do not see a reason why not do small jobs.
