On average, lies have a higher manufacturing cost (because you have to tread carefully and be more creative)

Hardly. It's much easier to throw bullshit at the wall than to clean it off.

In many public debates, people shovel outright lies again and again. In the time it takes for you to properly evaluate their lie, they've shoveled 50 more.

Also, their are lies, and then there is conceptual muddle that's not even false. Try cleaning that up. Conceptual muddle takes centuries to clean up.

, a greater risk (since getting caught will lower your overall persuasiveness)

Since when? With whom? Who has paid enough attention to keep track? This is one of the fundamental problem with public debates - nobody is keeping score.

If the people who already agree with you even notice, they'll likely shrug it off as a tactic, or just shift their attention to the next piece of bullshit supporting their views that they haven't yet seen through.

, and a smaller qualitative gain (while lies probably persuade more people, I suspect that they persuade less rationalists than civil debate and are therefore less qualitative overall)

It looks like you're suggesting that rationalists count more - somehow? Even if they do, they don't have the numbers.

Rationalists are good if you want someone to produce useful epistemic truths. If you want to persuade masses of people, probably not so good. They don't persuade, and their numbers are so small persuading them doesn't take you very far in the aggregate.

People use the Dark Arts because they're effective. Otherwise they'd be called the Dark Incompetencies.