Paying for something was hard when the internet got started, so things were free and paid for by the website's owner. Ads were a way to offset the cost a bit.

Paying for something was hard when the internet got started, so things were free and paid for by the website's owner. Ads were a way to offset the cost a bit. People got used to everything on the internet being free, also when paying got a lot simpler. The ads stuck as well though, as paying for a website is still not zero friction.