Can someone please give me a fact-based answer to the following question:

What is the reason why Delta prefers to offer cheaper award tickets on their partners' metal?

I don't understand this from a business sense, and I've been trying to figure this out for awhile after booking so many Skymiles award tickets on AS metal and AF/KLM metal.

Doesn't it cost Delta MORE money to pay for those seats on their partners rather than fill a seat on their own planes?