My understanding is that Against Malaria Foundation is a relatively small player in the space of ending malaria, and it’s not clear the funders who wish to make a significant dent in malaria would choose to donate to AMF.
One of the reasons GiveWell chose AMF is that there’s a clear marginal value of small donation amounts in AMF’s operational model—with a few extra million dollars they can finance bednet distribution in another region. It’s not necessarily that AMF itself is the most effective charity to donate to to end malaria—it’s just the one with the best proven cost-effectiveness for donors at the scale of a few million dollars. But it isn’t necessarily the best opportunity for somebody with much larger amounts of money who wants to end malaria.
In its ~15-year existence, the Global Fund says it has disbursed over $10 billion for malaria and states that 795 million insecticide-treated nets were funded (though it’s not clear if these were actually funded all through the 10 billion disbursed by the Global Fund). It looks like their annual malaria spend is a little under a billion. See https://www.theglobalfund.org/en/portfolio/ for more. The Global Fund gets a lot of its funding from governments; see https://timelines.issarice.com/wiki/Timeline_of_The_Global_Fund_to_Fight_AIDS%2C_Tuberculosis_and_Malaria for more on their history and programs.
The Gates Foundation spends ~$3.5 billion annually, of which $150-450 million every year is on malaria. See https://donations.vipulnaik.com/donor.php?donor=Bill+and+Melinda+Gates+Foundation#donorDonationAmountsBySubcauseAreaAndYear and https://donations.vipulnaik.com/donor.php?donor=Bill+and+Melinda+Gates+Foundation&cause_area_filter=Global+health%2Fmalaria#donorDonationAmountsByDoneeAndYear They’re again donating to organizations with larger backbones and longer histories (like PATH, which has over 10,000 people, and has been around since 1978), that can absorb large amounts of funding, and the Gates Foundation seems more cash-constrained than opportunity-constrained.
The main difference I can make out between the EA/GiveWell-sphere and the general global health community is that malaria interventions (specifically ITNs) get much more importance in the EA/GiveWell-sphere, whereas in the general global health spending space, AIDS gets more importance. I’ve written about this before: http://effective-altruism.com/ea/1f9/the_aidsmalaria_puzzle_bleg/
There is some related stuff by Carl Shulman here: https://www.greaterwrong.com/posts/QSHwKqyY4GAXKi9tX/a-personal-history-of-involvement-with-effective-altruism#comment-h9YpvcjaLxpr4hd22 that largely agrees with what I said.
If Gates Foundation is actually funding constrained I guess that explains most of my confusion, although it still seems a bit weird not to “top it off” since it seems within spitting distance.