Why does everyone assume this?

If a seat on DL is available to redeem in Flying Blue, then you can be sure there are seats available to redeem in Skymiles at their lowest rate. But the converse - which is what you are assuming - is not necessarily true

For there to be a seat available on DL for FB redemption, there has to be seats available at the lowest rate for redemption on Delta in Skymiles. But just because a seat is available to redeem at that rate on Delta in Skymiles, doesn't mean that there *has* to be any seats available to redeem on FB.

And this came up only a few days ago...

Lowest fare Delta award not showing up on KLM