There are a million channels for every one that gets any sponsorship, and there's an equivalent ratio for the pricing of these ads. The CPMs on sponsored content are usually lower than network CPMs until you cross over to the very popular channels, but even then it's really just shifting the production costs of a video ad to the youtuber and paying some premium for their reputation.

It's very hard to scale and again there are networks and agencies that aggregate channels for most campaigns and buyers. Anyway, Stackoverflow isn't producing their own content and the pricing mechanics of Youtube/video advertising is very different than display ads so this isn't really comparable.