Most left handers I know, me included, use the mouse with the right hand – leaves the left free for writing and typing. I’m not sure why right handers use the mouse on the wrong side tbh, guess they’re just physically less able.
I fit into that category, though it’s learned behaviour. First mouse I used plugged into the side of an Atari ST and the cable wouldn’t reach to the other side of the machine so I’d no choice but to learn wrong-handed. In hindsight I’m glad I did, I like having my dominant hand available for my primary input method.
As a race are we getting too lazy to move a mouse an inch to the left?
Some are just inept. Regularly I ask people to log in, and watch whilst they pick up the mouse, click the username box, type their name, pick up the mouse again and click the password box, type their password then pick up the mouse a third time to click OK, whilst I’m standing there quietly growing a beard to myself.