Someone who orders only a couple a year is probably not loosing a lot of money from not being able to or having a harder time getting them. Even if it's commercial, 5/y is probably more in the realm of a few individually customized setups.

Whereas if you have a complete product designed around an SBC, respinning that is expensive, and a few hundred to a few thousand units is an ugly point where that leads to really high per-unit costs.

Whereas if you make millions of a thing, the overall development cost is similar, but spreads over many more units. (EDIT: you also probably aren't using an SBC, for similar-ish reasons: a customized board is saving you money per-unit, and millions is enough to amortize the up-front cost of that and still come out ahead)

So the economic perspective IMHO makes a lot of sense.