Why shouldn't we test drugs and cosmetics on mentally challenged or severely disabled human beings, rather than animals?

Other things being equal, perhaps we might. But of course they're not equal. Our social morality -- the morality we live by -- is 'speciesist' in the sense that human beings -- whatever their mental or physical capacities -- are considered to be due special protection. If we were to seek to remove that protection, chances are that it would probably degrade our ethical sensitivities to the point that things went overall worse for non-human animals than they do at present. What we should be asking, rather than your question, is: Why should we continue testing drugs and cosmetics on non-humans to the extent we do, when we wouldn't dream of carrying out such testing on human beings with similar capacities?