uhh molten cast can have microscopic air pockets which can cause them to flex, bend, crack and the mold can be off depending on how old the mold is. forging, which I have only experience with steel not alum, strengthens the metal by not melting it but heating it to a point where it's malleable, making so a manufacturer can use a lighter alloy to achieve the same or more strength of casting, also the trucks are more precise because they use a die to cut and stamp the shape out. billet is a large block of metal, aluminum, that is put in to a cnc machine and milled down to a certain specifications reducing slop and weight.
too long; didn't read
forged trucks tend to be lighter and "stronger" but it's because they use an alloy that would have been too weak to cast but is lightweight and cheap enough to heat treat to make stronger. NOW TALKING ABOUT STEEL FORGED TRUCKS IF YOU FORGED TRUCKS OUT OF STEEL THEN YOU'D HAVE A TRUCK THAT'S PRACTICALLY INDESTRUCTIBLE. RONIN HAS DONE THIS BEFORE AND THE PRICE TAG IS INSANE