Because everyone likes money for nothing. The sex for free part is just extra.
But seriously, once people realized that after the split the sum is more than the previous value added together, there is no limit on possible forks. Why would be? It is like your stock paying suddenly an undeclared 10-15% dividend.
To use another analogy, bitcoin is a pimp. A pimp who gets new girls coming to him wanting to work for him. Thus bitcoin/pimp would get extra money from their earnings. The sex what the pimps get for free is just the topping on the cake.
As the thread title said, money for nothing, sex for free... Ain't we living in wonderful times?