Totally pragmatic analysis, based on locally-available data sources:
- Boys have more utilitarian, and typically shorter hair styles
- Boys have fewer clothing style and color-range options
- Because of the lack of diversity, boys' clothes tend to fairly universally "match", thus outfit creation as a skill is not learned until much later
- Boys are culturally directed to more active/aggressive play activities, which lose all cuteness as soon as one or more participants or bystanders are seriously injured
- Every so often, those cute girls you see, are really just well behaved boys giving fewer-than-normal gender cues to the observer[*].
Of course, a lot of these same things hold true straight into adulthood, but both men and women tend to become less "generally" cute to the average viewer, as individual tastes take hold.
[* As a young boy of the 70s who wore a comparatively wide range of colors (no pinks or anything "super girly"), and lots of overalls (which were pretty unisex and popular at the time), who had moderately long fro-hair, I was mistaken for a girl about 60-70% of the time in stores. While you may still think of specific boys and girls when comparing cuteness, I guarantee you've mistaken an unknown boy for a girl or vice versa out in public at some point – and I would argue that this is largely due to a perceived presence or lack of "cuteness" in the child. ]