This has been asked multiple times and it mostly comes down to the fact that it would make region specific pokemon (like relicanth, tropius, and many others) pointless. It would be nice to be able to do this with Ultra and Best friends, similar to PvP, but I don't think they'll do it because it would allow people access to pokemon without direct interaction with others. This reasoning is why their current trading system is based on being close enough to interact with the other individual.
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Will long distance trading (like long distance battling) be implemented?
They've shown they can successfully allow players to pvp over 100+ kilometer distances. So why not trading?
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In addition to the points raised already, it would also kind of make the Pilot medal pointless. You'd just have to add someone with 15k distance to yourself and do 70 trades. If you can find that remote friend from a facebook group or discord, that's barely more than an hour to go from 0 to gold medal.