Because clear roles on an org chart do not automatically produce a shared source of truth. Here is what actually happens at a lot of events, three weeks out.
The marketing committee has a registration count pulled from one export. The budget committee has a headcount estimate they built two months ago, before registrations even opened. The volunteer lead is scheduling shifts based on a guess pulled from last year's event. The logistics committee already placed the catering order off the old number. Nobody notices the gap until the caterer calls asking for a final headcount, and three different committee members give three different answers on the same call.
This is not a communication problem you fix by adding another weekly meeting. It is a data problem. Five people are each holding a partial, outdated copy of the same information, and none of their copies update automatically when someone else's changes.
The same failure shows up again at the door. A committee spends months getting the guest list, the sponsor list, and the VIP list exactly right. Then it hands the actual check-in desk to whoever's free that morning, armed with a printed spreadsheet. Manual check-in against a printed list runs 45 to 90 seconds per person once you count the "let me check the spelling" and "you're not on here" back-and-forth. That processes only 40 to 80 people an hour. Switch that same door to a mobile app with QR code scanning, or add a self-service kiosk for overflow, and throughput jumps to 240 to 400 people an hour (Nunify data across 200+ events). For a 300-person event, 60 to 70% of attendees arrive in the first thirty minutes. That gap is the difference between doors open on schedule and a line stretching into the parking lot.
None of this means committees are the wrong structure for running an event. It means the committee's real job is making decisions. Somebody specific needs to own turning those decisions into one shared system everyone can see. Otherwise you get five spreadsheets that agree with each other for about a day, then quietly drift apart.