Understanding the need for representative samples is not the same as actually getting representative samples. It's not a problem of scientists being ignorant or lazy or even underfunded. They know what they need, and they simply cannot get it. They try to make do anyway, but the quality of their work still suffers because of these factors which are out of their control.

Not all fields of inquiry are equally tractable to the scientific method, so they should not be treated with equal reverence.