The advertiser KNOWS the whole ad will be displayed, vs creating a 30s ad that most users don't see most of because they skip after Xs.
The ad is over almost before the user realizes it's started.
No more conditioning viewers to instantly enter the "how and when can I skip this" mindset. (Tangent: none of the full-window "here's an ad while the page loads" ads work on me; I'm conditioned to instantly hit "skip" and make a point of not noticing the ad itself).
Knowing the ad will be very short, viewers are more likely to not attempt to skip it because it will be done (or very nearly so) before they can do so.
If a user is going to see a given ad frequently, best to get it over with and not turn it into a harrowing experience of repetitive tedium.
There's probably a worthy study of ad effectiveness between the initial awareness of the ad and the cognitive turn against it. Get it over with before the desire to skip begins, or a "I hate this ad" arises.
Just make sure the viewers don't explode from the density & pace of content.