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.
I think at times that both parties seem the same is both get their millions, tens of millions and hundreds of millions of dollars to run their organizations and campaigns from basically the same people. Corporations, wall street firms, lobbyist, special interest groups, mega money donors etc. Neither can afford to peeved the moneyed interest off. If they do, then they may spend their millions on backing your opponent.
I think there was a time when both parties were close to being the same. I grew up under Eisenhower, back then the Republicans, conservatism, it was said that the GOP agreed with everything the Democrats were doing, only they want a bit less of the doings.
You didn't see party line votes back then, it was usually a mixture of both parties voting for and against any particular legislation. As for today's polarization, divisiveness and mega, ultra partisanship, I heard it referred to as avid sports fans rooting for their team against the other team. That folks have become mega fans of their political party. When the Phillies were good giving the Braves a hard time, they were referred to as the filthadelphia Phillies. Now the number enemy of the braves are the Washington Nasties.
It hasn't happened here, but in other cities where just being a fan of the opposing team got you beat up. That happened in Augusta, a teenager wearing a Miami Hurricane sweatshirt was beat up for wearing that sweatshirt. They're Georgia Bulldog fans down there.
Perhaps the avid fan metaphor is correct, maybe not. Regardless, each major party owes their heart and soul to these moneyed folks. Which very well can make each about the same. It certainly limits what each can and can't do. Perhaps we've been indoctrinated to view them differently. It's my party, my team vs. those evil Democrats or Republicans, those evil Phillies or Nationals. It us vs. them. although us and them are basically the same.