Prime video I don’t know so well, but YouTube was free without ads in the beginning, for something that is incredibly expensive to run. They had to introduce any monetization or shut down the service. They went with ads because 99% of users prefer that to payment. Later they gave the option to pay to remove the ads, only as an extra, because very few people are ready to do that.

There are some ad-free video platforms out there but they have a tiny fraction of the user base of YouTube. Most people couldn’t even name one, let alone considering using it, when YouTube is “free”.