Why do YouTube ads never buffer?
Ads on Youtube are (obviously) paid for: this is where Google makes the most money, as such it's in Google's best interest as a company to ensure that delivering ads works as fast as possible.
Additionally, ads are short compared to most Youtube videos, and the number of unique ads on Youtube compared to unique Youtube videos vary by quite a large amount (I don't have actual numbers, but lets say that there are 100k unique ads vs 9 million unique user-uploaded videos).
Because of the small amount of ad videos, they can cache them locally in data centers closer to people's homes and the video loads faster. They can't do this with all Youtube videos because there are simply too many and they are too long.