Because sex is a short-term goal, and the more heavily you optimize for it, the less you'll optimize for being accountable to the long term side-effects of dishonesty.
Our brains evolved to make decisions. Yes, they were constrained by a need to survive and reproduce. But those two goals are not the same thing. And the existence of those constraints does not actually preclude any other mode of operation.
You're being absurdly reductive when you conflate any motivation with a need to reproduce.
This is why your argument is nonsense. You're basically trying to define everything as sex. You're playing a silly word game so you can feel smug about this directionless and immature insight.