It's a good question, and a complex answer. The biggest reason is that tilting the fixture makes the fixture much more complex to build. It's already a losing proposition unless you're making 100s of knives. I'm only doing this because I can, and I'm a machinist, not because it's cost effective. You would either need fixtures of a mirror image, or an adjustable fixture. Even if the fixture isn't incredibly complex, it gets harder to locate your workpiece accurately when you start moving it out of orientation in the XY plane.
The second reason is chatter. Taking a large swipe with a face mill on a relatively thin section of material that's not constrained very well will inevitably chatter. The ball mill is programmed so that it takes a consistent chip load every pass. A consistently small one that doesn't exceed the workholding capability of the fixture or the rigidity of the workpiece.
Last, while one could possibly spend the time developing a complex fixture that held the part in multiple positions to get the bevels made more quickly, the other features of the knife need to be in the XY plane, and it will require even more setups to accomplish those. Such as the outside profile, holes, etc. When it's all programmed into the XY plane, I can hit the green button and go do something else while the machine is running with a very very low risk of machine crash. To do it the other way, with 2-3 minute cycles between each operation, I would need to stand at the machine and tend it.
The program I made I ran straight from FeatureCAM with no optimization at the machine (ie, there's some air cutting going on). I also programmed it at very low engagements to reduce the risk of tool breakage. It's a very stable program the way it currently is, even if not the most efficient. It has a 64 minute run time, and in that time, it's machining the primary bevel on both sides, the false edge cut, the entire outside profile of the blade, and the thumb serrations before coming back and milling off the workholding tab.
A rough finished knife every hour and all I have to do is load parts and hit go? I can heat treat the blades as they come out of the machine, or run the surface grinder, etc etc. Optimizing the mill program, or spending more time/resources on a fixture to reduce machining time, just doesn't pay off.
