@MCX01
Well, historically pearls have only ever been used as a decorations, we humans consider them pretty but it's not necessary merfolk do.
It have had very little practical use, so if the merfolk have a different sense of beauty then humans it's not unthinkable they would not appreciate pearls like we do.
It was only much later on when we learned how to use pearl powder as medicine, and again with a different physiology it might not be as effective for them or they have other better options.
That said, Trade would always be an option but its a fantasy manga so while its fun to consider the implications of the things the manga writers do, you should always take things with at least 7 spoons of salt.
Its not the most insane plot hole i have ever seen.