Every year, the cost per click goes up, which means you get less for the same budget. As the ads get more expensive, your target audience gets more ad weary, which means they pay less attention to the ads you create.
Paid traffic also disappears if you stop spending. So, whatever you're investing into building the perfect offer, landing page, and advertisement no longer provides value. To get anything out of them, you have to keep paying.
Organic traffic is much different. After you invest in one piece of content, it keeps bringing traffic to your website year in and year out. And, if you keep investing, you build a snowball effect - a compounding return on from your marketing budget.
Long story short, organic traffic is the only traffic source that keeps driving revenue long after you stop investing in it.