Because it is value extracted, not value added, and because the receivers of the majority of that value are in a position to extract that value by virtue of having historically exploited Botswana. So the larger part of the value will be removed from the country rather than that it will become part of the local economy it will become part of our economy.
That could be, but for the jobs, transportation, development and taxes they pay, right? The benefit to Botswana is somewhat diminished by having the diamond shipped overseas. But lets be honest: there is no real industrial/commercial value to this diamond at all; it is simply a trophy.
Diamond mining is highly mechanized. There definitely are local workers but fortunately it is no longer as labor intensive as it used to be (that's bad because fewer jobs, but it is also good because those jobs were super dangerous and very low pay anyway).