Whatever the reason, if they cannot overcome it, doesn't that make all their professional output similarly useless?

If no one can overcome bias, does that make all their professional output useless? Do you want to buy "philosophers are crap" at the expense of "everyone is crap"?

However, I don't agree with what you're saying; overcoming these biases is very easy. Just have an explicit theory which you use for moral reasoning, where results can be proved or disproved. Then you will always give the same answer, regardless of the presentation of details your moral theory doesn't care about.

That's the consistency. What about the correctness?

Note that biases might affect the meta-level reasoning that leads to the choice of algorithm. Unless you think it's algorithms all the way down.

After all, mathematicians aren't confused by being told "I colored 200 of 600 balls black" and "I colored all but 400 of 600 balls black

Which would make mathematicians the logical choice to solve all real world problems....if only real world problems were as explicitly and unambiguous statable, as free indeterminism , as fee of incomplete information and mess, as math problems.