Its not that simple. Its a good starting point for weight loss, but there's many contributing factors that can affect how one's body retains calories. Some people are lucky and have all of those contributing factors even out to where all that's left is a simple calories in/calories out equation, some are even luckier and just burn excess calories with no effort and never meaningfully gain weight, but some will fight hard reducing calories and increasing activity while seeing minimal improvement and end up extremely frustrated and burnt out because it isn't that simple.

This isn’t true, fyi. Different people have different bodies.