Have a look at the end of the video, you will see it's finishing process in snapshots to engineering specs. The main process you are seeing is only the initial forging before you use higher accuracy machinery for the final processing.

And with a total time less than it would take to machine the same piece out of an appropriately sized block of solid steel.

Forging is frequently used to get to an approximate shape, even in things as simple as forks.

Additionally - generally speaking - forged steel has better mechanical properties than cast steel. Example:

http://www.atcgroup.com.au/CustomCastingForging/TheDifferenc...

I was wondering why not cast it to a ring mold then finalize the shape ?

Were that to have been cast it would have had completely different metallurgical properties.

Molds can be significantly more expensive for low-volume parts, even for the same shape. There can also be problems with cooling of the shape.

If you view through to the end of shows how they are then machined into very tight spec parts. The completed product is anything but rough.