Multiple things here but the main problem is that many men register / see a female body any time after puberty has phsically strarted (curves / breasts) as "fair game" or even when they are just over a certain height ("old enough").
Plus the intense sexualisation of school age girls in our society including a general sociatal view of any girl over about 12 who is being accosted / groomed / flashed at / raped even as "asking for it". Girls from this age are presented in many situations as adult, manipulative, asking for it, sexually "knowing" or experienced, while the men abusing them are painted as innocents, or victims, how could they be expected to resist?
Massive problems to do with male entitlement, men thinking girls are "fair game" from WAY too young... And TBH they shouldn't be doing this stuff to older women either as it's shit, they often target younger girls as they will be more likely to give a satisfactory reaction (if the man even waits for one) > confusion, embarrassment, fear, and less likely to tell them to fuck off and call the police.
Although lots of people in the UK also thing that e.g. a schoolgirl aged 12 getting men shouting obscenities at her or following her is not and should not be criminal, it's for girls and women to somehow "manage" ie "society" that amorphous mass of generalised opinion, does not care really what happens to girls this age unless they are actually murdered. And sometimes not even then, if they are the "wrong" sort of girls.
And this is on men. It was common when I was young, seems hasn't changed. Society needs to stop seeing this as AOK and fine and girls shouldn't make a fuss about nothing and maybe it's banter or fun or can't they take a compliment etc.