it’s because goyim close the buttons so it’s probably chukos hagoy. furtheremore in the early 1500’s there was a cult of apikursim known as the sogrimchultzosim who’s symbol was a closed sleeve button. all the gedolim were mesaken to open all your shirt buttons. then someone was meorrer that there is a lack of tznius to open all your shirt buttons so only sleeve buttons were left open.
in all seriousnes people just want to give posters here something to talk about.