> If I go to a BMX page then show me an advert for a new helmet or whatever

> That's not how adverts work. Instead I get an advert for a new TV because I bought a new TV last week.

Whether that works well enough to pay for producing and distributing that BMX page depends on how big the market is for BMX-related products and how many different BMX pages are vying for the attention of people who visit BMX sites and buy BMX-related products.

For things with a large market and a small number of major sites that much of the market visits, it can work great. For smaller markets it might only work for the largest sites (if there are any). So you can easily end up with the biggest site or two getting almost all the interest-related ads, and the smaller sites only get ads for things that the general population buys.