I always found that odd, too. Neri must have driven to the hotel, because if you remember, Michael & Vincent took a helicopter to arrive at the meeting. But why Neri did not leave as protection to Michael is weird. Maybe it was a protection scheme. I'm just guessing here, but if something happened to Michael, then Neri could have run things for the Family. Vincent was not ready to head the Family at that point of the film. IMO, it's debatable whether Vincent was ever suitable to run the Corleone Family.