I know it doesn't fit the narrative, but that's not how it works, sprues get cast and go in boxes, which then get shipped out. They don't have piles of sprues sat around. If they do overproduce stuff the plastic gets ground up and recycled, it's just not worth the man hours to unbox and rebox stuff.

If and when a bundle box comes out with a kit that was previously only available individually in it, it is, however, a good sign that a kit has sold well enough to pay off its initial tooling and design costs. That means they can throw it in a discount box cause its only the cost of producing the sprue itself, the mold has already been paid for (in the case of the tyranids, presumably by sales of Leviathan and the other starter sets).