I have no idea why they chose a 7 day payout cycle, other than the fact who ever developed it sees it as work done in a work week, and thus they pay weekly. Kind of like a house builder. They get paid by the week, then when the job is done they do not get paid again. Now you bought that house, you do not keep paying the house builder for the house. If you took a loan to buy the house then you have to pay the bank to live there, and the government, if you paid cash, then you just pay the government to live there. But the house builder no longer gets any payment from you for living in the house. 

People like getting paid, they did at least understand that, and I can see why they chose a 7 day period. How often should the payouts be done on a post, how much computing power would it take to figure out 7000 post a day, 20,000 comments a day, and possibly 50,000 votes a day (all those numbers are just guesses on my part). 

To me the 7 day rule makes sense, it is a common pay day thing. Yes Steemit is an uncommon thing but people still want to be paid.