Because there is no market for it in US. In EU either. Its not profitable to ship it to neither of those, as it takes as much volume as the same amount of heroin, which is much more profitable and has much bigger market.
To the best of my knowledge, poppy fields are prominent in Asia and not here (Africa as well I think?) But as the other member said, likely the same reasons coca isn't grown in the US as well..
However, now days if it's cartel sourced, it's likely imitation (fent opiate mixed with stuff to imitate opium/heroin etc). Cause precursors are much cheaper to ascertain from their sources and cook themselves then it is to grow, maintain, cultivate, and smuggle opium/heroin from out east, therefore a much better return on investment.
if your ever on Reddit go to opiumgardening sub...