Because it’s not predicting power output. That’s what head flow says.
That’s why the same head may make 1.5 HP/CFM and that same head on a different engine will make 2.0 HP/CID.
It took me awhile to get my head around that.
It also has to do with some assumptions the math makes. One of those is it won’t account for someone building a 420 inch engine (this is why you have to account for bore and stroke in the math but it is weighted more on stroke and where you want torque and power peaking) with 273 heads.
The math assumes the engine builder has a grasp of what heads the engine needs to make the torque and power where you are telling the math.