Consider the war on drugs- not just a complete failure, but actively harmful, especially to minority communities, while rich white users continue about their business largely unmolested. You can get anything you want your first time in a visit to any American city inside of an hour. Just stop by a restaurant kitchen. Banning firearms would be about as successful. So the argument is, why criminalize a huge number of people who aren't criminals just because some criminals and mentally ill people use guns? It would lead to another drug war situation.

So I guess if you swing it the other way, the popular rhetoric around drugs is that they'd be better off legal and heavily regulated.

Pro-drug proponents are all for this. Why are pro-gun proponents against more regulation, especially lobbyists?

Regulation is a broad term for this context. I'm sure pro-gun proponents would be more than happy if the government regulated for product safety and quality and are generally fine with age limits and restricting sales to licensed dealers.