Because technically you want the server to teleport the authoritative proxy, not the autonomous proxy.
On actor Begin Overlap (Teleporter) → Switch has auth (authority): → Cast to character → As character → Srv Teleport (run on server).
That’s the proper way. The switch makes sure the overlap happens on the server instead of the client. The server event has the server teleport the authoritative proxy which gets replicated to all clients.
Here’s a teleporter reference from earlier last week.