It's always because of the ads.

Whenever you're wondering why the video is too long, or why the author is somewhat rambling around the point, or why they include those "like and subscribe" sections in every video (sometimes at the beginning, because you'd close the tab otherwise), the answer is always to find that sweet spot that accommodates the ads. It's also why channels tend to upload videos of similar length, regardless of the topic. Sometimes they have to cut the script short, other times they have to find a way to lengthen it.