I hear lots of talk saying that heavy gear and lenses cause people neck pain

I disagree with respect to this subreddit. I see a lot of what goes on around here, and I do not see much talk about that here at all.

Also it seems like most of the people posting here do not use a neck strap and instead use a shoulder or cross-body strap, or a holster.

So maybe this is not a great place for the target audience of your question.

how come so many people complain about this problem and are seeking to cull every gram from their gear? Are people too weak to carry a mere kilo around their neck?

Fatigue, strain, and other forms of discomfort are not necessarily the same issue as lack of strength.

But yeah, some people will feel pain in situations where you don’t, and vice versa. Different people are different. If you’re comfortable with your neck strap, that’s great for you. If someone else has problems with their neck strap, I’m pretty sure they aren’t just making it up. They’re just not as lucky as you. It’s okay for them to want to do something about it, even if you don’t need to.