The seemingly arbitrary gendering of buttons on men’s and women’s clothing probably isn’t something you’ve considered unless, like me, you’ve taken the perfect gingham shirt into the dressing room at a thrift store, only to be completely confused upon trying to button up.
The history behind this peculiarity is somewhat shrouded in mystery, but it’s generally considered to be a relic from the days when clothing was a lot more complicated. In the Renaissance and through the Victorian era, women—particularly wealthy women—wore elaborate items and often enjoyed the luxury of being dressed by a servant. In that case, it was easier for the assistant to have the buttons on the right side (when facing the woman being dressed) with the assumption that the servant was right-handed, as most people were believed to be. Men usually dressed themselves, hence the buttons on the other side.
Of course there’s some question about whether this makes sense as a lasting convention if such a limited number of people could afford maids to dress them, but the nobility were the tastemakers when it came to fashion and it’s reasonable to think that those traditions trickled down and have stood the test of time only because no one bothered to change them.
Other possible explanations also cite the fashions of hundreds of years ago when men’s clothing often included weaponry. A right-handed man could pull his weapon out with his primary hand and unbutton with the secondary. The hand-in-waistcoat portraiture of the time somewhat supports this idea, with the right hand tucked into the open flap.
Another weapon-related theory: standard fighting position meant facing the enemy with your left side with the shield. A shirt with an overlap from left to right meant that a foe couldn’t aim a sword in you through an open slit.
On the flip side, maybe babies are the reason? Women usually hold a child on their left side to free up a dominant hand and a shirt that opens on the right makes breastfeeding easier.
Several other theories exist, including ones about horseback riding, Napoleon's Napoleon complex, church etiquette, and what might be the most unsettling of all—that the distinction was made during the standardization and early days of mass-produced clothing and was meant to reinforce sexist attitudes by forcing women to button with their "inferior" hand.
Most likely though, it’s because Marie Antoinette and the like needed help securing their bodices.