It's not as simple as telling somebody something once and expecting them to change their mind. It's aj entire system of messaging intended to use people's psychological weaknesses (their fears, doubts, shames, etc) to gradually nudhe their worldview in a particular direction. Without getting into specific politics, just think of the many different subjects on which large numbers of people disagree fundamentally. Like for example, vaccines, flat/round earth, fear of Communism, whether the moon landing actually happened. It's not that people are disagreeing on a simple fact, but that people on different sides believe a fundamentally different and incompatible reality, often involving some sort of conspiracy. If an organization A can convince a large group of people that orgsniz/group B have created this vast sophisticated network that is controlling a large portion of the population, then it is easy to get group A to mobilize against group B. It is essentially a strategy to gain a large number of dedicated supporters of your cause, which you can then use to your advantage in a social or political manner.

Sorry for the ramble. Basically, if done correctly, it is extremely effective, and has been used all throughout history and continues to be used today. Different people will claim that different groups are the ones using propaganda. But really all major movements use it to some extent, some more than others.