To the GP's rhetorical point, if you /really/ are wondering why you shouldn't, then you aren't actually happily married.
To your original point, no, there would be no point to being honest if being dishonest made it easier to get ahead. But it would be very naive to presume that being dishonest makes it easier to get ahead. You need to have cunning to get ahead by being dishonest, and one false move can screw up a long period of well executed dishonesty. Are you personally equipped to pull that off? Better than those who've been practicing their whole lives? And beyond that, the psychological and sociological ramifications of it will catch up with you too. Who do you think you'll tend to be surrounded by if you take such a strategy? What kind of life experiences will you never get to experience as a result of that? You don't have to look very far to get answers to that. It's not pretty.
For some people, pure wealth or fame is worth any degree of dishonesty, by which I mean the negative effects of it simply do not cause them difficulty. But I think the far more likely case is that our societal discourse minimizes the difficulty it does cause. When peace decays and order crumbles, it doesn't make warfare (literal or figurative) any less expensive or unpleasant. It just makes it harder to avoid.
