Mostly because it's extra money for the fifth continent. A not so insignificant amount in many cases. Remember that we are starting these from funky places and are willing to come up with wholly obnoxious routings for a number of different reasons. Some, for miles. Others, to travel to 30 different places on a long ticket. But most people who just want to leave for three weeks or a month to do some traveling are not going to want to bother leaving from Timbuktu to save a few bucks or routing 6000 miles out of their way to reach top tier. If someone wants to visit a limited number of places in Asia and Australia, there is no reason for them to pay for an extra continent. Especially if they have little use for other benefits from it like airline status.