A simpler explanation is that they just love to gamble.
Essentially, they bet on ending up in a higher position of the pyramid.
Many of these schemes even have weekly "meetups" aka services akin to those of religious congregations. These usually include an iconic leader who admonishes the minions for the coming week and encourages them to cut out any family or friends who dare to question them.
I was thinking more in terms of a long-term commitment that provides enough benefits on its own to offset the time and effort spent by each participant on spreading it to others.
Like biological parasites, the long lasting ones seem to have optimized the level of damage and chaos they cause, along with maintaining protection mechanisms (lobbying, political donations, corruption) that enable them to continue to survive and be incredibly lucrative to their founders.
It doesn't stop people joining, unfortunately. Knowing that Sue, the really successful seeming Super Global Ambassador Tier leader at the presentation on Saturday earned less than a shelf stacker does not stop some people figuring if only they join Amway they'd be doing better.
In Ponzis and Illegal MLMs, there's either no product or a sham product to disguise money from new recruits paying off the lineage.
Legal MLMs are legal because there's a more legitimate product that accounts for a larger share of revenue.
But even for-profit corporations that need to grow to survive share some features with the illegal variety, it's just that the real value of their products is a larger portion of their total revenue.
Maybe these schemes really sell hope to those that need something to believe in, replacing religion on some level. Except in this case it's economic hope, the belief that if they work hard enough they will be rewarded.
I've noticed a trend amongst participants. The people at the top are straight up psychopaths looking to exploit human greed for their own gain. But the lower level minions are usually simply poor. They have very little to lose, so why not try it? Ultimately they usually end up poorer, but there's always the story of Bob and Betty who made a million dollars last year and came from nothing to be top sellers.