Why are oxygen requirements based on pressure altitude? The FAA's oxygen rules in §91.211 are based on pressure altitude. Assuming our lungs are like our engines and wings and dependent on density altitude, why are the oxygen requirements not based on density altitude?

If the answer is practicality, then would it not be wise to be more conservative with these requirements on hot days and more lenient on cold days?

Living in Colorado, the oxygen requirements are almost exactly what is usually required to transit the front range of the Rockies without an oxygen bottle, which leads me to believe practicality played a part. If density altitude were used instead, on a hot day you'd be unlikely to remain within the regulations and still transit the area without hitting anything made of rocks.
The air in the lungs is in pressure equilibrium with the cabin, but not temperature equilibrium. It's always at body temperature. So the only variable that can affect how much oxygen is available to the body is the pressure of the outside air - thus pressure altitude, not density altitude, is physiologically relevant.

That said, if the density altitude is high it may mean you are hot and dehydrated, and the deleterious effects of those factors may be additive with those of hypoxia.