It's not based on cluelessness, people are aware that there are ways to circumvent the screenshot notification. The vast majority of people don't install third-party apps to save their friends' snaps, so content you share on Snapchat is still drastically less likely to be on someone's phone forever than if you were to send it some other way.

I was never about security. They use FOMO to create a daily habit. When you post, people see it and react quickly, which further strengthens the addiction.