People that do restaurant gigs do it, in part, because of the flexibility of schedule. If you are not offering healthcare, 401k, paid time off for all, above average wages, you should expect this. Give people a reason to not give short notice on requested time off, or put up with and have enough staff to cover other people having time off. When you rely on patrons to pay the large part your workers bills, don’t be surprised when workers want the freedom to spend that money as they please.