This phrase is commonly used in a joking manner on
social media to express that someone is attractive with a dose of subtlety, shyness or
embarrassment for having such an opinion.
The sentence is in an unfinished form for the same reasons, while the continuation is easily guessed - ...sexy / hot / handsome / fabulous / majestic etc.
More emphasis on the attraction being a contrarian opinion is commonly expressed by including a 'but' before 'why is bro kinda...'
The sentence being unfinished also alludes to the speaker knowing what they're about to say is unhinged, or losing their
train of thought and speech as they gaze at the object of attraction, or as they bite their lip.