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YouTube warns it might make your viewing experience worse if you don't turn off your ad-blocker
(www.businessinsider.com)
This is a most excellent place for technology news and articles.
I think there's a simpler reason; if you embed ads as part of the video linking to a specific timestamp becomes a nightmare. One person might have no ad, another might have a 30 second ad, and a third might have 5 minutes of ad. Attempting to link to a time after the ad would give three different timestamps, and loading that timestamp could give you the clip you want or could drop you right in the middle of an ad.
To solve that issue you'd need some way for the client to determine the "true" timestamp, but then you're also giving ad blockers a way to determine where the ad is so you're back to square one.