Before revision: The reason that extrinsic mortality appears to do so is that it increases competition within populations: it is more accurate to say that harsh environments create situations of high competition, in which people are more likely to adopt fast dstrategies to maximize their chances of reproduction, than it is to say that individuals in harsh environments adopt fast strategies because otherwise they would die before reproducing.
After revision: The reason that extrinsic mortality appears to do so is that it increases competition within populations: it is more accurate to say that harsh environments create situations of high competition, in which people are more likely to adopt fast strategies to maximize their chances of reproduction, than it is to say that individuals in harsh environments adopt fast strategies because otherwise they would die before reproducing.