Generally, you would expect to see casting for most tooling structures: The cast shape is a "known" irregular, highly stressed FOUNDATION for all subsequent machining and assembly. A welded-together assembly will stretch, move, flex under welding, and carry unknown heat stress levels after each welding seam. The net is a more expensive total process - because even after welding, you still have to align the fabricated walls and fixtures and footings and sides together and machine all of the same surfaces already cast at one time in one single mass.
Consider even a hand tool like a drill: cast it in plastic because of the complex shape, rather than gluing together the different individual pieces. Clamp the final halves together mechanically.