I thought it was the other way around? Keyword ads on search results are by far the most profitable since you have intent right there ("what is the best lawn mower"), whereas the vague stalker-ish amazon ads about buying another one of the thing you just bought (or just trying to guess "well homeowners in their 30's might be interested in a lawn mower") have way lower value.
Especially on Google Search results, the top ads for brand names are usually for the first search result anyway and people click on them because they're first, making google lots of "free" money as people have to protect the top spot for their own brand