Why do websites impose payment minimums?

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I write for different websites that pay, have a Google AdSense account, an Amazon Associates account, and an account on Lulu through which I publish my books. All these sites have a minimum amount that people must earn before they can get paid. Some websites have a low minimum, while others (like Google AdSense) have a high minimum. People won't get paid until those threshold amounts have been reached. If I have something in any of my online accounts mentioned above that I have already earned I want to get that money... I don't want it sitting there for months or years, and potentially never going into my bank account at all just because the minimum may never be reached. When people have a job, they get paid every month or every two weeks, or even every week, no matter what. The same should apply to paying websites mentioned above. No matter how much people have earned they should get that money every month. Why does there have to be a minimum for people to get paid (especially a high one)? I never understood that. I think it's a bit unfair to impose that, especially when the minimum amount is high, because if people earn something, but never get to that minimum amount, they will never see what they have already earned, which is rightfully theirs.