What actually happens
Nobody decided to let this happen.
Work happens at a site. Hours end up in a text message, materials on a paper ticket, photos on somebody's phone, and the note about what actually changed on the job in somebody's head. Later — usually much later, usually by whoever is still there — all of that gets reassembled into a job record and an invoice.
Every step of that reassembly is a chance to lose an hour, a detail, or a billable item. The billing goes out late, it goes out light, and the person doing it is normally the most expensive person available.