Most castable alloys will do for this casting, as they are generally quite conductive. You do need to take care of surface finish to get all the sand off, and you will have to mill the actual device mounting areas to get them flat.
The copper plugs are generally added by a press into a machined hole, and then the whole assembly is surface finished in the area to get the smooth flat surface required.
The reason most use an extruded heatsink is because you just cut to length, and the as extruded surface is generally smooth enough to use without further machining if used with a thermal pad or heatsink compound. As well you can get a very complex high surface area in the one dimension from the die, using a minimum of material, and can easily have slots for mounting hardware. A casting can do this, but the moulding will be complex and prone to cracking during casting as it cools faster with large surfaces compared to the thicker areas.