I'm not saying it's not as good as a scientific survey, I'm saying it results in no information at all about the strength of correlation one way or the other.

E.g. one could seek to find a 6' preteen and 6' adult to construct a counterexample to the idea height is in some way correlated with age. Doing so gives just as little evidence of what the strength of correlation is as seeking to find a 5'9" preteen and a 6' adult to show the correlation is positive or seeking to find a 6'1" preteen and a 6'0 adult to show the opposite. I.e. it doesn't follow one can filter the search as they please and then assert that's what the correlation of the unfiltered searches should be assumed to look like. In all 3 cases of positively correlated, negatively correlated, and not correlated we'd expect to be able to construct an example which says whatever we want to say - that isn't the same thing as sampling what the actual correlation usually is.