Why is blood donated but sperm is sold?

Why do sperm banks pay but blood is only donated and not sold, if anything it should be the other way around right? Blood Is far less enjoyable to give, when you donate sperm you at least bust a nut right? With blood you get poked with a needle and get blood sucked out of you with a tube like some kind of lab experiment, is it cause of the requirements being more strict or something?
Why is blood donated but sperm is sold?
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