Normally we're able to hide the spaces where blocked ads would go. On some web pages, however, you may see them as blank areas on the page. Yahoo Mail is one we hear about a lot, but it's not the only place this happens.
The reason those spaces are there is that we weren't able to reformat the page to use that empty space―and the reason for that is that somehow it's an outlier. That is, something about the page is very much different from the majority of web pages we know how to reformat.
There are millions of ways to design a website. We know about a lot of them. If we tried to cover every possible exception, we would be doing almost literally nothing else but figuring out how these exceptional pages are laid out. Remember, we're trying to do this with computer code, not the human eye and brain.
Even if we could somehow manage to allow for every possible design choice, sites change constantly. We simply couldn't keep up, and if we tried to, it would be a maintenance nightmare!
But really...wouldn't you rather see a blank space than an ad, anyway? :)