The Saha World is just a part of the many many world systems there are out there in the vast tenfold directions of the three periods of time of empty space, of which Sukhavati is another one, of which Vaiduryanirbhasa is another one, of which Abhirati is another and so forth...So why the need to even try to fit one world system into another world system? That's akin to asking why isn't Mars part of the Earth instead of accepting that both are within our solar system yet separate entities?
Another answer if one can think of is already in the Sutras, look at the nature and quality of Amitabha's vows and Sukhavati? With the antithesis quality and nature of the Saha World, for instance retrogression, reversibility and unassured mortality are hallmarks of our conditioned world, can Sukhavati which has features of of non-retrogression, irreversibility and enduring longevity be manifested as per Amitabha's Vows here?
Another one: there can never be 2 Samyak Sambuddhas in the same world system simultaneously as per the Buddha Dharma for reasons you can investigate on your own. Our Saha World is already under the current Dispensation of Sakyamuni Buddha who has manifested Mahaparinirvana whereas Amitabha Buddha hasn't, where He is still teaching and transforming in Sukhavati. And let's not forget that we have Maitreya in waiting...
West of here, east of there? What do these mean... west of my country would mean Indonesia but it can also mean India, the Middle East, Africa and so forth?
Just conventional language used to describe something plausible to our conceptual mind but at ultimate reality, what do these directions mean?