The same reason that people advertise gambling websites that acquire money from those who don't understand the mathematics of negative expectation:

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Greed.

For many people, putting money into an obvious ponzi is a form of gambling.  They are "betting" that they will be one of the lucky ones that can get their profits out before the ponzi collapses.  Meanwhile there are others that fail to understand the mathematics and take risks beyond their personal risk threshold without realizing it.

Sounds a bit like "dice" sites, doesn't it?  Many people "bet" that they will be one of the lucky ones that wins at the dice site.  Meanwhile there are others that fail to understand the mathematics and take risks beyond their personal threshold without realizing it.