Maybe in the cosmic sense you are, in that they have a giant pile of money, and you contributed a few pennies to it, but this is not how accounting works. Your transaction and their ad budget are separate things.
Also, advertising does other things than tell you to buy something, and it doesn’t always take the form of banner ads. Apple, for example, does a ton of brand awareness advertising. Affiliate marketing often targets direct transactions. Maybe your goal is to simply start a relationship that might someday lead to a really big purchase.
Often, in the era of SaaS, people advertise to existing customers. Apple does this—they have a TV service and a music service and a cloud service.
There are plenty of reasons for them to advertise after you bought the original product.
But your original point was that customers bought the ads. Maybe they didn’t! Maybe they were given funding by a VC firm and the company decided it wanted to build an audience. Maybe they want to advocate for a political issue.
I think the biggest problem with your argument is that it has tunnel vision and sees advertising as this one dimensional thing, when in reality it takes many forms. Plenty of those forms are bad, but it is not as simple as “I bought a product, now I never want to see an Apple ad ever again.” Many businesses (Amazon, eBay) make most of their money off of customers they’ve already advertised to that they advertise to again and again.