E.g. because of much longer guaranteed support/supply life, which is a must if you are trying to run a serious production. There might be some other reasons too.
The underlying components in the Pi Zero also have a long "guaranteed" support/supply life. You don't have to rely on the Pi foundation for your components, or the board itself (as I pointed out in another reply in this thread).
My primary point is that the cost of a GPU these days is very close to zero, so I see little benefit in picking a platform that doesn't have one if you need that capability.
And no, even in the best of cases where you have great software support from the vendor (ha! as if) and a great reference design, adding a GPU to a design is expensive because of both software and hardware R&D. unless you have an integrated CPU/GPU combo but then you're back to the problem of dealing with Qualcomm/Broadcom.