Earlier quoted context omitted.
Because, as mentioned higher up, you might actually want to support ad-based sites. This is an easy way to do it with no effort. I use uBlock. But I approve of the existence and success of ABP and similar. People differ.
You can support ad-based sites with either tool by whitelisting a site. uBlock requires you to do this explicitly for each site you want to support through ad revenue. I suspect most novice users will never do this. ABP is now using their user base as leverage to extract payment from some sites to show ads by default. I don't think that is a good idea at first blush, but there may be arguments for the practice I have…
That is, I do not believe that other solutions have a very clear way for advertisers to determine what is and isn't acceptable; for the sake of argument, this puts aside the fact that some unacceptable aspects of ads should be common sense (loud noises, pop-unders, fake download links, etc). Basically when an adblocking user reaches the site, they have no means of communicating why they dislike the ads on that site - most of the time, it's just a blanket block that users never think about. So even if you really like a website and would love to support it through the ad revenue, your current options don't send a very clear signal as to what was disruptive and dislikable.
Again, I'm not saying I fully agree with this, but I think it's an argument that has some value, and reflects the greater difficulty with advertising in general; it's a one-way communication venue, and the few times that we as viewers are able to get a signal back to the other side, it's usually ignored until legislation gets involved. (See the CALM Act and it's overall ineffectiveness [1]) Basically there isn't really much control over what advertisers actually do, and even Truth in Advertising laws basically have no teeth except for extremely blatant falsehoods.
Adblocking is the first major way that viewers have been able to push back against advertising, and I'm with that 100% since I just think it's pointless in and of itself - it's wasteful in my opinion how much infrastructure exists out there just for the sake of providing advertisements. But I also understand that likely advertising isn't going away, so having some method to signal back clearly and directly to advertisers what is wrong with an ad is good in my mind. Said signal having some teeth to make sure they get the message is also important, else it's not much better than before. But not giving any indication as to what's wrong doesn't really help either. (I'm not blaming anyone but stubborn advertisers here, but I think it helps us as activists if we can make it 100% what we don't like about specific advertisements instead of just doing a blanket block).