Why do people keep saying this? I don't know any scientists who make such claims.
When politicians are trying to push an agenda one way or the other, they like to erect a glamour around the science making it appear settled. But then if you ask the scientists themselves, the answer is always "it's complicated" and "there is more work to do on this topic."
This is true and all, but at the same time it would behoove us to differentiate when someone is an expert or "stakeholder," or someone "using the lay press" to "politicize" an issue!