Making lots of money out of stock trading is rather easy in the presence of clear and relevant individuating information about all entities that are being traded. So is any decision making. That does not mean that in real life, "decision making" or "stock trading" are simple problems that we shouldn't be concerned with.
The irony is that there is strong prejudice that every prejudice and bias is harmful and in a major way. This actually remains to be seen.
To follow from your example, investors in many index funds do not have access to individuating information yet are making profits. Which means their perception of the market is at least somewhat accurate. But some investors who do have access to special information (and are not caught as doing insider trading) cash out much bigger. Often they do no better or worse too. You get too actually evaluate outcomes of each investment (or hiring) system.
Unsurprising at all. The cure is either blind auditions or extended trial periods. Or impartially designed tests.
Tricky question is who is supposed to bear the cost.
That is if they problem really is of big magnitude and worth solving in the specific case. Which is done by measuring the magnitude and outcomes rather than detecting presence of prejudice. Then investigating the causes by hard analysis rather than politically biased guesswork.