point_direction in GML has one specific purpose. The math.atan2 function in Lua is multipurpose, as is everything else included in Lua's relatively minimalist standard library. The reason why there isn't a function for everything is because you'd need to keep a huge dictionary of functions whenever you need to find out how to do something. It also makes it so you can't have as many variable names; in this case, because that function exists, you can never have a global variable named "point_direction".