Because news articles etc are used primarily for promotion. Which means their main intended target is people who don’t know much or anything about the game.
For your average Joe 1.5b isk means nothing. But they can easily understand 20 dollars.
So a battle or heist involving xtrillions may as well be in monopoly money (which it realistically is. Except that most people have a better understanding of what monopoly money is worth)
Because of plex we do have a conversion rate between irl money and isk. (Even if it’s only a one way conversion in practice) which allows them to put battles into real world terms which more people will understand.