A trial commits in advance to a target number of participants who complete a set period of follow-up. Dropouts and missed visits break that commitment, and the trial cannot finish until the shortfall is resolved, either by recruiting replacements or by accepting a smaller completed dataset.
A missed visit is not free
Suppose a participant skips a scheduled check-up. The measurement that visit was meant to capture is now missing. Staff must contact the participant, arrange a new appointment, and decide whether the gap can be filled late or must be recorded as missing. None of that work existed before the visit was missed, and the trial's data is still incomplete until it is resolved.
The delay is not caused by the dropout itself but by the trial's fixed requirement for completed follow-up. That is why retention has a direct effect on the calendar, not only on the budget.