Many, many people don't think it's a bad thing. As an American I look at things like GDPR and the proposed copyright law and completely understand why a sovereign country wouldn't want to be part of the EU.
You may not agree, but it shouldn't be mindblowing that reasonable people hold this view.
No one knows if it is actually a bad thing to leave yet, give it 50 years.
For some reason people read a couple of articles and become experts on really complicated, multi faceted, non deterministic systems and decide to expound their opinions on those around them.
You may not agree, but it shouldn't be mindblowing that reasonable people hold this view.