A variation of the Terminator paradox (can't transport metal unless it's covered in flesh). In this case, you can't transport metal unless you wear it fashionably.

Seriously though, it was a gameplay decission from what I've read. I assume they realized how punishing not having portals would be, specially if you died far from your main base and had to recover your corpse, but didn't want to completely eliminate the need of logistics when it came to gathering materials. But, you know, people circumvented it with mods or "planeswalking" (having a world with a prolific seed to just gather materials and once you have them you swap to your main world with said materials right at your base so you can build and craft in peace).