I wouldn't say that everyone wants to have sex with underage girls, but the data suggests about 90% of men might fantasize about preteen girls, at least occasionally, and something like 99% of men would be interested in girls under the arbitrary age of 18.

The real question is not why men would be attracted to young women - the reasons for that should be pretty obvious for a species that reproduces itself through men having sex with young women. The question is who pushed the narrative that there is something wrong with normal sexual attraction, why they pushed it, and most amazing of all, why whole cultures ever accepted such an absurdity.