Taxi service is not the same in every destination. Sometimes you cannot simply walk outside and hail a cab. You may always arrive and choose a transportation company to provide the service inside the airport, but without arranged transportation, you will not only be bothered by the timeshare bandits at the exit, most of the times you will pay more and will waste valuable time in yet another line to arrange this.

After waiting to go through Immigration, waiting for your bags and waiting to go through customs, people just want to get out of the airport and have an assured transfer to their hotels. It's simply a matter of convenience.