The reason high-end phones are $600 at all and not thousands of dollars is the number of units the enormous fixed costs for research, design, development, and certification are spread across. Having an extra model that takes two SIMs means another model that carriers will want to test, another model the testers can lose in a bar, and another model salespeople have to help the customer decide between. The cost of that is high, and vendors are making the (appropriate) decision that it would get high enough that they couldn't profitably build and sell it.

The prevalence of dual-SIM phones in the third world is mostly due to the cost difference of on-network vs. off-network calls in the developing world outweighing the cost of a phone that supports two networks simultaneously or the (personal economic) cost of swapping SIMs all the time.

Competition between providers in the US and Europe means these cost differences are small compared to the higher R&D costs for cutting-edge phones and the inconvenience we'd be dealing with swapping SIMs.

For a demonstration of how high the costs of making a smartphone can be, look at Vertu's Constellation Quest, which even at $8500, wasn't expected move enough units to justify the cost of a unique platform over their parent company Nokia's Symbian.