Google shows you ads based on what you type in so you are directly telling them what you're looking for at that moment. That's a pretty good position to be in as an advertiser. Some people will do what you do and go right to a specific search site, but others will start that process at Google.

Amazon can do the same, but the user is probably already looking to buy, although with less breadth of category coverage. Amazon is also more of a closed ecosystem, which is less flexible for advertisers but probably better for conversion.