Proof of that is the fact that we can't even get water to all of these people.
Not sure how good an analogy this is. I suspect it would be moderately expensive (i.e. on the order of a few billion dollars), but possible to deliver a liter of clean water to nearly everyone in the developing world. The problem is, that solves their water problem for about 12 hours, if you're being generous. Solving their problem long term is the hard part. With vaccines, you only need to visit the people a finite number of times, and then you're done. Given that HIV is sexually transmitted (and assuming that the vaccine can be given relatively young), you wouldn't even need especially frequent visits to immunize the next generation of people.
Not sure how good an analogy this is. I suspect it would be moderately expensive (i.e. on the order of a few billion dollars), but possible to deliver a liter of clean water to nearly everyone in the developing world. The problem is, that solves their water problem for about 12 hours, if you're being generous. Solving their problem long term is the hard part. With vaccines, you only need to visit the people a finite number of times, and then you're done. Given that HIV is sexually transmitted (and assuming that the vaccine can be given relatively young), you wouldn't even need especially frequent visits to immunize the next generation of people.