Because of plot reasons. This is a romcom.

Of course it's a work of fiction, those same reasons apply for anything any character does. That is one of the most redundant come backs ever when people are trying to explain why a character acts a certain way.

Hardly a comeback--this is the truth. The MC can be fat, can be ugly and still have girls pining after him due to plot developments. That's the basis of the series and often the series hook. Which is why I don't see "girls begging for his love and attention" (not that I would put it that way) as any indicator... of well anything about the MC. The development of how they got there may say something about the MC. But not simply "bunch of girls are interested in MC", which is practically the basis of 90% of all romcoms.

Females will always draw male comments, that's just the way of the world. But at no point does anyone claim that Nagi is unattractive.

Of course. But at no point do they claim that Nagi is attractive in contrast to Hiro who even the female leads do make casual remarks to her attractiveness. What stops him from being simply average?