Do they not do that? I remember watching one or two videogamedunkey videos that included ads and he specifically scripted the video around when they would appear.
I think the creator can do that, so some use it creatively. Otherwise, the algorithm seems to pick the "most rewatched" places, so the most interesting part of the video will be interrupted by an ad, regardless of how jarring it is for the viewer.
I believe both options are available. But inserting ads manually is more work on content creator's part, so they often use algo provided.