another reason besides the obvious one that I recently realized, is reliving something similar like when you were at school and for the first time realized some teacher had a nice rack. she was probably not in any sense hot, but maybe she leaned over a desk and her breast pressed against her shirt so that they became more visible. and for the very first time in your young life, your brain screeched to a halt and went: "wow, boobies!"
you were probably around that age and probably fantasized about the teacher all classes after that. in real world nothing ever happened (unless you were the future president of france), but that didn't stop you from thinking of that semi-attractive 45yo french teacher or whoever it was, right?
now if you wanna tell that story as a fantasy where it DOES go further, it's gonna be a shota story. if you replace the MC with an older one you lose the whole point of it, so you're stuck with shota.