Why is the "not all ___ are like that" argument so ineffective?

Well, being able to provide a single counterexample to a general statement (an "All" statement) does refute that statement.

For example, if I claim that "All swans are white" and you say that they are not all like that and produce a single black swan, you have proven my statement to be false.

But, when people make claims they usually don't say "All".

So, they never claimed "All", and when someone retorts denying the "All" claim - they are denying a claim that wasn't made.

Even when someone does say "All", they probably don't really mean "All", but rather some high percentage that can't be refuted by a single counterexample.

For example, if someone claims that drinking large amounts of alcohol often leads to liver damage (notice they didn't say "All") and then someone else responds that their old uncle Jake drank a gallon of scotch a day and lived to be 135 years old without any liver damage. That is not a refutation. The original argument wasn't a universal claim so a single counterexample is not a refutation.

You know after reading the answers that have already been posted I have to agree with all of them and even if you put the word all into the sentence it still gives the general belief that not all Women, Men, Dogs, Muslims, Christians so forth and so forth are the same or do the same thing or act the same way as others of that grouping may act for no two are exactly the same.

I do believe that using that statement is a very good rebuttal because in truth, not everyone is the same, nor do all people have the same beliefs or motives which drive them.

Assuming it's a true statement (ie: not all pitbulls are like that), I think it's a perfectly good argument. You say, "Muslims are terrorists" and I say, "Some are, but not all Muslims are like that." Seems like a valid argument to me. It's true, it provides perspective, and it counters the assumption made by the first statement. In fact, I'm not sure what else you could say, unless you could say, "No Muslims are like that," but that would be another untrue generalization.

- You need to provide more context. That is a good counter argument to "all ___ are like that" (which is usually considered sexist/racist/__ist). Example: That martian is rude, so I don't like martians - Not are martians are rude.

However, it is an illogical counter-argument if the general members of that community are not part of the argument in the first place. Example: "That pit bull bit my son - Not all pit bulls are like that"