The offer gets signed. Now someone in HR types the same name, start date, and pay details into payroll, then benefits, then an IT ticket. The ticket sits in a queue behind whatever broke that morning. The manager gets a calendar invite and nothing else.
Every step depends on a person remembering to trigger the next one. The tax forms go out when someone thinks of it. The equipment order happens when the ticket surfaces. The day-one agenda exists in the manager's head, if it exists at all. Nothing is technically broken, which is exactly why nobody fixes it.
The real cost isn't HR's hours, though those add up fast. It's a new hire you're paying full salary to produce nothing for a week, while they quietly revise their opinion of the company they just joined.