Why? My recommendation to every single person is to install an adblocker right now and never ever turn it off again. It's such an easy thing to do which immediately significantly improves your quality of life.
The same reason that if I go into a cafe to sit down for a while, I'll buy a drink even if I'm not particularly thirsty. The cost of having a table in a downtown location for you to sit at is far higher than the cost of the beans and water; a drink for a place to sit is part of the "implicit deal".
Websites cost money to run, so I think it's only fair to accept a reasonable amount of advertising. But I don't think being silently tracked for just happening upon your website is fair, so I block trackers. If your advertising works without trackers, I'll see it just fine.
To extend the metaphor: Malls often have common areas where you can sit and just hang out. They also often have cafes with tables. The common areas are often nicer than the cafes -- less loud and less cramped. If you like common areas better than cafes, by all means, sit in the common areas. But if you do sit in a cafe for some reason, you should buy something, even if it's just a bottle of water.
By a similar reasoning -- if you prefer websites with no ads, then by all means only go to websites with no ads. But if you do go to a website supported by ads, I think it's part of the implicit bargain that you be willing to view ads that are presented in a reasonable manner (where "tracking you across the internet" is not reasonable).
I don't like the concept of tipping -- I think people's salaries and/or bonuses should be determined by their managers, not random people who wander in. But if I go to a restaurant where tipping is expected, I tip, because that's the implicit agreement I made when I sat down.
I've always found it super depressing that in so many towns and cities virtually the only places where people can just hang out are commercial in nature.
It would be so much nicer if the towns and cities invested more in to providing such places themselves (beyond merely having a park here and there), where you didn't have to be a walking dollar sign to someone in order to participate.
Do you also make a point of buying some percentage of the products advertised to you? After all, isn't it ethical to support the advertisers that are supporting the websites that you read? It costs money to run those ads, after all, and without the ads the websites wouldn't run, according to your reasoning. Or would you rather externalize the cost of supporting those advertisers to others while you get a free ride?
Tracking is yet another thing you can avoid to some degree by using an ad blocker (and, better yet, by disabling javascript).
Finally, the best reason to use an ad blocker is to opt-out of the advertising ecosystem to make it less viable. We have to find a more ethical way to fund media than supporting this manipulative, privacy-invading industry.