I've never ever heard anyone say diet and exercise don't work. Is this something a real person has said to you in real life? Not on the internet?
I hear this all the time to the point where I don't even mention looking at a person's diet and exercise because I expect they'll be shot down
It's something I've read a few times. Most of the studies I've seen say it's effect is modest at best. At least if it was a drug there wouldn't be enough evidence for it to get FDA approval (especially with all the "side effects" such as injury)
Arguably this is a limitation on how the studies were done, as nobody is going to pay for extensive studies since you can't patent exercise.
Bodies are all different. Diet and exercise can work for one person and not another. But the type of person who wants to believe all fat people are slothful, all unemployed people are lazy, and so on, will invent in bad faith the straw man argument that we think diet and exercise do not work at all. They're not worth engaging with.
I have known at least two people, who while not making the claim that it doesn't work, do make the claim that it doesn't work for them.
To me this is just subconscious denial/defending of their food addiction.