I've had this thought before too. Like, someone somewhere must be clicking on these ads but I've never met them. The closest I've found is my dad who clicks on the google search ads (before I blocked them for him) but he isn't exactly an active internet user. He might open youtube once a month for one video when hes trying to fix something. Noone is making much ad revenue off of him.
A large chunk of advertising isn't to get you to buy their products right that second. The goal is brand recognition so when you are ready to buy the products, you think of them first.
Like, why does Coke still need to advertise? Literally everyone knows what they sell.
Advertising revenue is generated when people watch ads that are running on videos. This revenue from ads is shared between YouTube and the Creator - thus empowering Creators to directly profit from their work.
Companies pay money to Google for showing ads to you.