So as a content creator, I can give you some insights into why this is. Keep in mind this will change eventually, and doesn't apply to every creator.

Simply put: if I make a video that's eight (8) minutes long, or longer, I can change where the mid-roll advertisement is placed. If it's less, I cannot. I also get less adverts the shorter the video is. If it's ten (10) minutes or longer, I get more and more adverts.

That's why when my wife and I watched a video about how a certain food stuff was made (I think it was a type of pasta), they made it 10m 30s long and used b-roll footage of pasta moving along a conveyor belt for 2-3 f-ing minutes.