> This isn't wikipedia.

No, it's a forum where the norm is that if you are going to make factual assertions where you aren't the source, you should include the source.

> If someone is so concerned about sources, they can include it in the reply after the parent.

Which is just making everyone else do your work for you. It's fine, if you want to make assertions and not include sources, you'll either be ignored by some number of people, or depending on how different the assertion is to their worldview, possibly downvoted.

If you're interested in having a discussion, sources help. If you're just interested in putting your mark down and saying something because you have the urge, then they don't really matter.