Back on topic please.
Animation is not about being detailed. Animators can't fuck around doing character development all the time so they condense character design and personality together. In the case of breasts, they are a signifier of adulthood more than a signifier of female-ness. Animators then would rather just have a character wear a pink shirt and have long hair to show that a character is a little girl and use breasts to show a character is a grown woman. If your show doesn't have anything to do with what happens in between then there's no need to worry about it. A good example of this is Phineas and Ferb. Candace was not a grown woman, their mom was. By giving Candace breasts it would have messed with the design and not add anything to her personality. Later on the show got Vanessa, she was more off model and she behaved more seriously, so the animators felt more need to show off her "grown up" traits. In other words, you take signifiers and keep adding them to a character until it is easy to figure out who they are and how they are likely to behave.
Now, there is this weird line on most people's heads that separates characters that are ok to sexualize and ones that are not. Breasts are a big thing, there's a lot of baggage attached to a girl with breasts. So, since most cartoons are not about developing sexual tension between characters, most animator would rather not deal with making characters with a lot of sexual signifiers.
Of course, I'm just talking about the practical side of animation. When it comes to rule 34 and hentai, then it pays to be more detailed. Character development has already been done as most rule 34 is about sexualizing already established characters, or development doesn't matter since the point is about getting off and discarding the whole thing afterwords.