I mean, you can say "'because it's fiction' is a shitty excuse," but that's basically what it boils down to and you either accept it or not. It's like asking why there's two gates in Brotherhood. In my opinion, the gates, Truth, the principals of alchemy, et al are so far above the characters as to be supernatural, and such things should be allowed creative leeway.
A better question than "why doesn't alchemy flow both ways" (because doing so would, as I said, mean they wouldn't be able to use the real world thing, which is pretty core to the '03 anime) would be "what does Earth get from deaths in Amestris?" It's an unanswered question, but not one that's explicitly denied nor an actual plothole.