To be honest I've seen a couple advertisements that I watched the full ~5 minutes. Both were game trailers that were really interesting. I would have rather had a "click here to view the actual trailer video instead of the ad version" button though. I was sold after the first 2 minutes, but couldn't see the full name of the game until the end.

Game trailers as ads?

Over 95% of the stuff I watch on Youtube is gameplays, collagehumor and tech reviews, yet all my ads are diapers or clothing softeners. I've no idea why their algorithm sucks so much for my account.

Unless they figure you’re already going to buy it and don’t bother targeting you?

I've seen multi-minute prerolls on videos <1min long.