The answer to your dilemma is in the premise: all these companies needed to do was to provide concrete assurances that the testing would net participants a spot

The answer to your dilemma is in the premise: all these companies needed to do was to provide concrete assurances that the testing would net participants a spot in the front of the line and due compensation. That would mean spinning up resource supply chains and infrastructure to make sure not just testing, but actual administration, of the vaccine would happen there. But because of the neglect and exploitation much of Africa has experienced from major powers, that would have turned testing into a prohibitively funding- and time-intensive prospect. African leaders aren't stupid, they recognized that. So without those concrete assurances, any widespread testing would almost certainly have been yet another example of exploitation, where the spoils go north. And for what? Most African countries are dealing with COVID perfectly fine. Europe and America are the ones where the suffering is disproportionate to the norm. This is not a case of Africa suffering for its lack of desire to play ball with Western powers; it's one where Western powers killed hundreds of thousands of their own citizens unnecessarily. If there's any injustice, it's in that the people paying the price in our countries are the people who get screwed over by institutional neglect - sometimes, sabotage - time and again.