I think it has to do with the fact that to an outsider who hasn't experienced sexual objectification or sexyness, both look the same but they aren't. If something is sexy it causes arousal. If something is sexualized then it is usually only relegated to being a sexual object .

Women seem to understand this difference more so then men due to experience. It's a human urge to feel sexy, to know you can cause your partner arousal and that you are desired, however it's also a human urge to not be relegated. Women are relegated to sexual objects while also wanting to be sexy so they learn the difference. Many women don't obviously but a lot of them seem to know it subconsciously and it causes many men to be confused.

To an outsider after all both look almost the same. If a man finds breasts sexy or if a man sexualizes breasts, they'll do similar acts. They'll compliment, admire and ect. However their priorities will differ, a man who sexualizes will relegate, enforce his belief on her body. It's different from a man who just finds it sexy, where he has a desire but isn't enforcing it onto another.

It's likely that because many men aren't sexualized or feel sexy themselves that they don't notice it. You kinda have to be told or experience it. When you feel sexy, you aren't being relegated and it's like a liberating feeling of expression. When you feel sexualized it's like you are being trapped. Trapped to limited expressions because that is all there is of interest.

it isn't a lack of sympathy but likely just experience because I seen many women make this same mistake as well. Usually those types of women have their ideas of sexyness align with how they are relegated. It makes them not feel objectified despite others classifying it as such. In turn they themselves face similar confusions when people claim they are being sexualized.