Men’s buttons are on the right to make it easy to do up with one hand for right-handed people.
When buttons were invented, only the wealthy wore them. Wealthy women had maids who would do the buttons up for them, and so the buttons are on the left so that they are easier for someone facing you to do up.
That’s just one theory.
Another theory is that it isn’t about the buttons or the ease, but actually the direction of the topmost fabric.
Men having a sword on the left side could draw the sword with the right hand without getting caught in the shirt.
Women breastfeeding with the baby on the left arm could more easily cover up when the shirt opening over the baby. Women riding a horse in sidesaddle style would also avoid the wind blowing their shirt open when buttoned that way.