the major part of the reason is that only one in ten people in the entire world are left handed, so by and large the majority of the world does not compensate for such a minority.
since the OP used First Person shooters I can giver a strerling example here. until the 1980s most firearms manufacturers did not make a serious attempt to accomodate lefties, so weapons constantly ejected spent casings to the right. I don't know if they were THE first, but the IMI adopted a new firing mechanism on the Galil Rifle that allowed a user to rotate the firing bolt 180 degrees with the use of a simple allen key. this change was made when a study showed an unusually high concentration of left handed shooters in the IDF.
today many firearms have adopted either this swappable firing bolt, or have taken some other clever method of not harming Left handed shooters with spent shell casings. The P-90 Personal Defense weapon for example ejects spent shell casings through the bottom of the grip.