What’s wrong with not including interests on your resume/CV?

There's nothing explicitly wrong. Including interests has no downside, and plenty of upside. So why wouldn't you do it? It gives resume readers and interviewers an opportunity to understand you better, and there's always the chance your interests will overlap with theirs. If that happens, it's a huge boost to your chances (for instance, if you and your interviewer are both avid mountaineers).