Is there no land to build on left as everything is agriculture?
The issue is that our the lack of cheap land actually made our agricultural sector very competitive. They need high yields to be viable, so they made the investments (equipment, r&d) to have extremely high yields.
More than 80℅ ends up being exported.
But you're absolutely right; it's a mess that has been left to fester for far too long. A previous law, that was struck down, allowed farms to expand on the assumption that those expansions would at some later time result in less nitrogen. That of course didn't happen, but massive farm expansions have been driven through that loophole. Farmers have also been misled by banks wanted to fund those irresponsible expansions.