Salesmen used to drink large amounts of organophosphates to show farmers it was harmless -- now we know that class of peaticides cause serious neurological disorders.
Or more importantly, where is the evidence that exposure to much lower dosages of DDT causes more harms than millions dead of malaria?
However, the mechanisms that drive this are more complex. A grossly simplified version goes like this: Since the population that's hit worst has no buying power, no pharmaceutical company is interested in developing a vaccine or an effective, cheap treatment. Malaria is very much a poor peoples illness due to a variety of contributing factors: No access to effective treatment, no access to mosquito nets, no proper hygienic conditions, no enclosed living spaces that keep mosquitos out.
Just spraying DDT on every swamp in Africa is not a solution to the problem. DDT accumulates in the food chain, mostly in fat and gets fed to the children you're trying to protect via the mothers milk. It is linked to an increased rate of stillborn and cancer. It's really not the kind of stuff you'd want to use if there's any alternative.