Might sustainability and geological context play a role? I thought the the Chinese islands were build on coral reefs and existing small islands. I also wonder about differences in currents and even requirements based on future use. A small military base to extend your empire probably needs to be less stable than a place you want to build a city on that should last forever.
Comparison to the Palm and World islands in Dubai strikes me though.
Projects like this usually have multiple goals, not least of which is to guarantee a baseline number of jobs with a desirable skill set within the country. Out of the tens of thousands of people that will work on this project, most likely fewer than 100 will work on it from the start of their career to the end. For the rest, it will be a year or two of their entire career.
Comparison to the Palm and World islands in Dubai strikes me though.