The spreadsheet was fine until it was not. Someone replies to your message, you note it in a row, then a companion turns up that nobody counted, and a meal you did not ask about arrives at the table untouched by a guest who cannot eat it. Manual tracking fails quietly, and you only find out when the vendor calls for a final number.
An online RSVP form is designed so that the mechanics carry the counting for you. Each answer is structured data on the moment it arrives: attendance is a value, companions are attached to the guest, meal choices are a pick rather than a note, and the totals at the bottom are already correct before the deadline.