Ask each person on the receiving end of propaganda if they are seeking to be lied to. The number that say yes is the amount of demand.
Those markets used to be broken. They are often still broken until the law catches up. https://en.m.wikipedia.org/wiki/2013_horse_meat_scandal
Turns out if a market isnt well regulated scammers will absolutely run amok. Look at any posting about Amazon here and how much of the comments are just saying it is just fraudulent garbage. When a expert actually does look at something, that's often what they find https://news.ycombinator.com/item?id=10508494
The big difference is that it's not illegal to spout whatever propoganda you want, and more than that, it sometimes even has government backing and I can't refute you. If you sell me a fucked up car it's obvious to me after a while, if you lie to me about what's going on in say, Ukraine politics, how can I know you are lying to me? I've barely even heard of the place before the war happened. I just have to take your word on why it's important we need to send these guys billions of aid as opposed to all the other countries currently being invaded.
Unfortunately this is not reality. There are many reasons why statements that are untruthful, or at least tangential to the truth like rage-bait, are paid to be distributed.
Sometimes domestic political parties seek to spread misinformation because it will drive their core voters to the polls, or convert on-the-fence voters to their side.
Sometimes nation-state actors seek to spread misinformation to cause chaos and disrupt national unity.
Maybe an industrial lobby is seeking to obfuscate or misinform about products and their benefits (see the sugar industry's public campaigns for 'low-fat' foods packed with sugar as beneficial to your health).
In all three of these examples, the parties supplying the information have vastly more resources to spend on spreading the information than anyone does to combat it, even if it is a net-negative on society, because there is money to be made or power to be had. Business interests can sustain their propaganda as long as they are making money, possibly indefinitely. National political parties can sustain their propaganda as long as there are major interests that benefit from the party's actions, typically also indefinitely. And we all know the kind of resources a nation-state can deploy.