The market is specialized and a very narrow slice will really benefit of the combination. It just so happens that we are in that slice...
Hence my conclusion - it is not a profitable/wise move to unify these two, from the manufacturer point of view.
Excellent observation.
We are seeing a tiny bit of exploration into Arranger/Synthesizer combination with some companies (mainly Yamaha and Casio) adding features like arps and real-time ADSR controls to their entry level instruments.
I had a Yamaha PSR-E443 for evaluation, and I was very impressed at the surprisingly rich tones I could get out of it with the two (assignable)real-time control knobs that let me filter and adjust the sound just like an analog synthesizer, including being able to get the filter to self oscillate.
I'd hoped to see these features (with more advanced control) on the mid-line and top-end Arrangers, but so far, only entry level for some reason...again, I think it's to test the market.
Baby steps, I suppose.
Ian