On the premise that that parties are the same, I'd say the arguments for that are based on the idea that sameness is the problem, but I think it's a really just a symptom. If they were truly the same, then how is there so much angry polarization?
Your thoughts about [unethical] people using that as a campaign tool are revealing. In addition to that, certain media business models are built around monetizing the polarization, and they have been very successful - in terms of making money. They have also been a key part of the very destructive ideology of "noble insurrection".
If there's a binary choice that folks can make about politics, it's between fighting and fixing. Fixing is a long, hard slog that may never really get anywhere (winning is not really a goal). But fighting is excellent entertainment and more fitting with human nature (or so it appears). With fighting you can win! And then lose... just ask don't ask Trump, who has always been a winner - even if he has to lie about it.