Why is arterial pO2 normal in carbon monoxide poisoning?
First, let's think about what pO2 means:

pO2 is not the same as "oxygen concentration." pO2 is the partial pressure of oxygen, you can think of it as proportional to the "number of oxygen molecules per unit area leaving the solution" (or colliding with the side of the container). There is also pO2 for the atmosphere, which is going to be approximately 21% * 760mmHg = about 160 mmHg. Partial pressures are often a convenient way to measure gas concentrations, but that only works when there aren't any other interactions happening, so it's not valid for blood.

When blood comes in contact with inspired air, there are >100 mmHg of oxygen molecules hitting the solution and &lt;100 mmHg of oxygen leaving, so there is a net diffusion of oxygen from the inspired air to the blood. If the gas in the alveoli and the blood in the lung capillaries have the same partial pressure of oxygen, there is no more net transfer of oxygen, they are at equilibrium.

Hemoglobin can't do anything to cause the blood to carry more "partial pressure" of oxygen. What hemoglobin does is to "hold on" to oxygen molecules. These "held" molecules can't leave the solution, so they don't "count" as much towards the partial pressure. Hemoglobin instead allows for a higher concentration of oxygen at the same partial pressure of oxygen. Another special thing hemoglobin does is to change its affinity for oxygen in the acidic environment of metabolically active tissue, releasing O2 at a higher partial pressure than it was initially dissolved at.

If you pumped plain saline through the lungs, it would get to about the same partial pressure of oxygen, but it wouldn't have as many oxygen molecules.

If not enough fresh air is entering the lungs, or if the air entering the lungs is low in oxygen, you would expect the blood pO2 to drop, but this isn't what happens in CO poisoning.

There is also the issue that what actually happens in CO poisoning is that affinity of hemoglobin for oxygen increases, but this is actually irrelevant to the pO2 question. The oxygen tightly bound to carboxyhemoglobin (CO-poisoned hemoglobin) doesn't "count" towards the pO2.