I think there's some kind of lore reason, but I'm not a fan either. There are mods out there to remove that limitation. I recommend them highly if it bothers you.

I use mods to remove that limitation. Might remove what boat's were ment for, but to much sea storm's effects my motion sickness.

You build a road to the swamps to transport the metal with carts and/or boats. probably thats why you cant tp with them. After all, its a survival building game, no rush anywhere.
Using mods to avoid the limitation is just cheating yourself.

I personally like the idea of having it not allow TP with metals and probably the same reason why the devs don't do it. It basically removes the use of the boats/carts. But then again I get some people want to play how they want, so it should atleast be an option by default.

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).

I dont mind it at all and I like it. If you dont, add mods.

For me, it actually meant having to build in all the biomes, or at least positioning myself well for minimal travel time. I'd convert ruined buildings into local smithies. It made moving on to the next biome into an entire adventure, not an occasional jaunt.

Still, I can understand the frustration and I do like myself a big, centralized base. So I can't decide if I like it or not. Maybe it'd be better if more was done with boats so you could use them for transport more easily.

It was an argument between the two main devs. One wanted unlimited teleporters, the other wanted no teleporters at all. This was the compromise.