Some airlines offer an auto-check-in option. I tend to not want to use it.

Part of the reason for having a check-in process that isn't automated for all ticketed passengers is so that the passenger manifests can be more easily/quickly checked and reconciled with who is on the flights and also for ticket settlement/reconciliation purposes. It probably also reduces the number of people that would otherwise get marked as having flown and lose ticket value despite not having flown on the flight as originally ticketed.