It’s because they appear to be something they’re not.
They’re usually friendly and fun and do all sorts of employee retention activities like arranging go karting and such…
They seem like they’re there almost as union stewards, to try and help retain employees and ensure you’re treated well by management. This is not the case. They’re there to protect the company from lawsuits originated by you. This means that they’ll apply rules and such in ways that are not usually beneficial to you.
They’re actually really helpful if you have issues with a coworker! However, you need to remember that despite how friendly they seem, they’re not actually in your corner, they have their own agenda.
So the simple answer is that they aren’t bad at all, but it can feel bad if you thought they were your friend.