I suspect it's because the erosion of the elements from land happens extremely slowly and barely detectable trace amounts, but the oceans is where they accumula

I suspect it's because the erosion of the elements from land happens extremely slowly and barely detectable trace amounts, but the oceans is where they accumulate over the course of billions of years. I think the real answer is that these elements (sodium and chloride) easily dissolve in water, and there's no process that removes them from the ocean, so they linger and accumulate.