In a nutshell, forging forces the metal into a shape, whereas casting pours it into a shape. Due to the nature of metal and it's molecular grain structure, forging typically yeilds a stronger part.
Casting simply pours the metal into a mold.. not necessarily bad, but often there's more porosity with casting, a random grain structure, and thus a overall weaker part.
As for a forged slide starting from a casting, it's possible. A manufacturer could start with a rough casting and forge it into it's final shape. But I would think that in the end, it still wouldn't be a strong as a slide that started life as a rolled billet.
These are all rough generalizations - there are so many variables that can affect the end strength of a part, like the alloy used, heat treating and annealing, machining steps, and so on. I'd suggest doing some Googling on metallurgy, steel, etc.