Because they are categorically different problems.
If you are sexualizing teenagers in your show, you're feeding into the nasty part of the fanbase that have pedophilia tendencies. It's not to say that anyone who finds these character attractive is necessary a pedophile or even that narratively it's always wrong to do so (if your writing teenage characters, it would be a little weird for them to never have any attraction to other teens, for example), but more concerning is the open admission that high school characters can be subject to sexualization even when it serves no narrative purpose, which can play into how people perceive high schoolers in real life. Ask any girl, she'll probably tell you they had some guys that leer or harassed them sexually while they were just teens, and shows that sexualize teens normalizes that behavior and they shouldn't do that because...that's bad. Like, morally.
Whereas if your just writing a generic, by the numbers plot and characters, like Hbomb's criticism of Ruby having no character except cookie cutter heroics....well, that's not good in terms of writing, but I don't see that having any social ramifications. Ruby being boring af doesn't incentivize any men or women to act creepy, you know? It's a waste of a character, but it's nothing more than that
Edit: If your problem with sexualizing teens in a show is that it's a worn out trope, then that's fixable, because I'm sure there are some artists that sexualize teenagers in all sorts of new and innovative, creative ways