Explanation
Everything downstream of the hook only matters to the people the hook kept. A video with a superb second half and a vague opening is, in distribution terms, a video almost nobody saw. This is why experienced people write the hook first and build the rest to pay it off, rather than writing the video and then bolting an opening on.
Good hooks are specific. They name a person, a number, a mistake or a moment precisely enough that the right viewer recognises themselves and the wrong one keeps scrolling, which is a feature. Vague openings underperform not because they are boring but because they fail to select an audience, and an unselected audience does not stay.
Because the hook is short and self-contained, it is the perfect thing to test. The same video body with four different openings is four experiments at almost no extra cost, and the difference between the best and worst of them is routinely larger than the difference between a cheap production and an expensive one.
The common mistake
Warming up. Any version of "hi guys, so today I wanted to talk about" spends the only seconds that were guaranteed to you.