It’s because the “algorithm” looks at the average watch time and promotes content that people tend to watch to the end, so they incentivize creating content that has some punchline at the end, or that is just so confusing that you spend the whole time trying to figure out what is going on so you’re less likely to swipe away. If your mind can easily make a judgement about what you’re watching, you’re more likely to decide it’s something you don’t want to watch, and then swipe before the end. This counts against content in the algorithm.