Like it or not, many popular websites and services online gain income from advertising. By using them while blocking their ads you're basically getting something for nothing by gaming the system. It's probably against their ToS (but who reads that right?). It probably says: We serve ads. If you don't like that don't use this site.
One might say that ads are distracting, annoying, resource heavy, and sometimes downright malicious. I think so too. However that is essentially the "price" of ads, and this price is known in advance. Blocking ads for these reasons while continuing to use the sites is not unlike eating at a restaurant and leaving without paying because you think their prices are exorbitant: It might seem morally justified but it's theft because the prices were known in advance.
I think one of the main reason people don't dine-and-run (yet install adblockers) is that it's hard to do a moral wrong so close to the source, e.g seeing the waiters, being there in person, risking an unpleasantness drama, etc. (This is well known psychological phenomenon which name I can't recall.) In contract installing an adblocker is fast and feels totally unrelated to the 2nd party. It makes it easy to feel like you're fighting the bad guys (ads and the terrible ad-tech) and forget whose revenue you are withholding for your own benefit.
Personally I resisted installing adblockers for a long time though eventually I caved in (ad-tech got really bad). I honestly don't know what the solution is.