Last year the U.S. experienced something that hasn’t definitively occurred since the Great Depression: More people moved out than moved in. The Trump administration has hailed the exodus—negative net migration—as the fulfillment of its promise to ramp up deportations and restrict new visas. Beneath the stormy optics of that immigration crackdown, however, lies a less-noticed reversal: America’s own citizens are leaving in record numbers, replanting themselves and their families in lands they find more affordable and safe.
You have to file taxes with the US, most people with US citizenship living outside the US don't actually have to pay anything.
As for why to keep filing:
renouncing your citizenship is difficult and expensive
it's hard to avoid the US
Let's say you have no plans to ever live in the US again. Does that mean you never want to visit friends or family you left behind? Does that mean you'll never go to a sporting event, concert or professional conference in the US ever again? If you're flying internationally, will you always be willing to pay extra and do extra work to avoid being on a plane that makes a stopover in the US?
For most people it's a few hours of work, and/or a hundred bucks or so once per year to keep their options open and avoid major headaches.