A main reason youtube is so successful as an advertising product is their detailed metrics. Virtually every sponsor will want to see each video's metrics which show retention during different parts of the video. It would be the same as putting the ad at the end of an hour long video; if they see a huge drop off where almost no one sees the ad, they may decide the creator didn't fulfill their end of the contract, or pay them proportionally to the retention during the ad.

Why would a sponsor pay for an ad if they know nobody will watch? Its not magic money out of nowhere; the sponsor has to come out net positive or else they stop sponsoring.

Thing is, the way a company determines if a sponsorship is working is by using offer codes. If no one is using an offer code, the company is going to assume that that sponsorship isn't working out and might terminate the deal.