Why are vampires always lolis? Getting bored of that concept.
Yeah, I am also not a fan of the concept. I think an age disparity between body and soul can be interesting, but this is often used as a moral excuse to show off the bodies of child characters, which I personally find distasteful.
In some vampire stories, there's an in-universe explanation for it at least. For example, in Hellsing, vampires are created by the bite of a vampire but only if the victim is a virgin. Otherwise they become a ghoul. This means that many vampires can end up being children or preteens (though none of them in the original manga actually were).