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The main reason is to keep SBI structured as a program that you use to support other content creators that you believe in. It also helps the word-of-mouth spread more quickly and encourages people to give SBI shares as contest prizes (since you get shares when you sponsor others).
As a social experiment, we would like to bring a sustainable basic income to everyone on Steem.

That a noble goal, will see how it goes over time ;)

Yes, only time will tell.