Nevertheless, even in Japan, it's not very lucrative for the farmers. Perhaps they should switch to farming mixed shellfish and kelp too.
I mean it is definitely eaten, just not that much of it. And I doubt consumption could go considerably higher, even if the price were artificially lowered by subsidies.
Now as you say there may be other reasons to grow it, I don't know enough about it.
And while you're right that the current market for these sea vegetables isn't massive, chefs are starting to work more and more with them which is shifting cultural acceptance.
Traditional seafood gets a lot of its "Fishyness" from the sea vegetables at the bottom of the ocean food chain. So you can create a lot of beloved seafood flavors just using sea vegetables. As ocean populations decline, traditional animal based seafood is going to get more and more expensive and I predict there will be a demand for a cheaper way to experience those flavors.
And as chefs get better at utilizing these ingredients and if more and more farmers and companies start to get involved with growing these plants, i'm sure industry marketing initiatives will ramp up just like you see ads from the "Peanut Growers Association".
https://www.fosters.com/news/20200107/unh-research-cows-fed-...