Evidently, the answer is that the DX7 was already cheap to produce. Engineering new stuff and spinning up a new assembly line would probably cost more than they would save. Remember, market segmentation is often about price discrimination. It's wrong to assume that consumer goods are priced at cost-plus-margin.
For what it's worth, they're actually totally different machines under the hood. They share the same main CPU, sound chips, and D/A section, but those are about the only similarities. I think you and GP make great points about product differentiation/price discrimination though. I think this explains Yamaha's motivations.
https://en.wikipedia.org/wiki/Price_discrimination