There are a few (well not a few things) you forgot to take into your calculation:
Most interest rates are now compounded monthly or even daily, your calculation is not even compounded at all.
$15 represent payment per month, think Home Loan, every $15 you repay on the $300 loan does not equal to $15 principal, it's principal + interest = $15. Why do you think most loans are 15+ years?
The game company is a business. A bank is also a business. So you think you pay money to SE, where is that money going to? They get reinvest to make more money. Maybe you don't care that your money get reinvested

but most of us do, even the billionaires do.