Specially in mobile, if you don’t update your app yearly, it will probably be removed from the store soon. Google and Apple can randomly Review your app and decide that it failed review even if it passed in the past. And fixing it to pass review is often not trivial and can take weeks of work.
Also, with each new version of Android and iOS, your app can stop working or become outdated. The platform API changes frequently.
Finally, if you use any third party libraries in your app, vulnerabilities might be found in the that you’ll want to have patched ASAP.
Oh and of course, you need to pay 99$ a year for Appstore access.