In the Web Vitals initiative, we want to make sure all of the metrics we define have a user-facing component, and directly affect some aspect of the user experience. While long tasks *can* sometimes affect the user experience, they also might not affect it at all.
In general, the reason to avoid long tasks is to ensure your pages are responsive, and we are working on a metric to help quantify overall responsiveness on a page. See: https://web.dev/responsiveness/
That being said, for individual sites these types of low-level metrics often can be very helpful in diagnosing underlying performance issues, and we recommended each site consider which metric would work best for their specific needs. You can read more about these types of metrics here: https://web.dev/custom-metrics/