Casting isn't necessarily "lower quality," it just has different properties. It's also much older (by thousands of years) than machining or pressing, but you can't get to the pressing or machining step without having an ingot, which comes from the casting process.

The main drawbacks of casting are you get a hard, but brittle product with (generally) uneven quality. There are processes (like annealing, though I don't know how you anneal a massive component) that can solve these problems, but all iron/steel is "cast" at some point.