Why does everyone call it "auto attack"?

^ Pretty much.

You don't need to do anything to continue attacking. Just click on an enemy, walk away, come back tomorrow and Cait will still be shooting her rifle without you doing anything.

Admittedly it doesn't happen as much in League where people move so much that continuously firing is ever a reality, but it was a term invented in other games and the shoe still fits.

There's a bit of risk, though. Sometimes.