Por que o tempo de consciência útil é apenas alguns segundos em grandes altitudes, quando consigo prender a respiração por muito mais tempo ao nível do solo?

When you are breathing, oxygen ($\mathrm{O}_2$) and carbon dioxide ($\mathrm{CO}_2$) are exchanged between the alveoli in your lungs and the environment. This gas exchange is based on difusão, which means the partial pressures of each gas involved will move towards equalization:

Henry’s law states that the amount of a specific gas that dissolves in a liquid is a function of its partial pressure. The greater the partial pressure of a gas, the more of that gas will dissolve in a liquid, as the gas moves toward equilibrium.

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As long as the oxygen partial pressure is higher in the environment, your blood will gain oxygen from breathing. But if the oxygen partial pressure is lower, you lose oxygen from breathing. Therefore, holding your breath at sea level gives you more time until you run out of oxygen than breathing at 15km altitude, where total pressure is about 10 times lower than at sea level. Furthermore, as John K pointed out in the comments, holding your breath at sea level allows you to sentir when you need to breathe again, because of increasing $\mathrm{CO}_2$ levels, where breathing at high altitude does not feel different because $\mathrm{CO}_2$ can still leave your system.

This principle also allows oxygen masks in aircraft to function without creating higher pressure. Since (more or less) pure oxygen is created, the oxygen partial pressure is much higher than in the surrounding air despite being at the same total pressure.

The situation in a high altitude depressurization is different because:

1. The air in your lungs is now "FL500 air" - i.e. the pressure is about 0.1 atmosphere. This means that the partial pressure of O₂ (ppO₂) is about 0.021 atm, instead of 0.21 atm. Oxygen will rapidly diffuse out of your blood and into your lungs, and your brain will very soon not have enough oxygen to function. (You can actually manage with a slightly lower partial pressure of O₂ - scuba diving mixes are often considered "hypoxic" once they're below 16% O₂, since your can breath a ppO₂ of 0.16 atm and function normally. But note that this is 8 times greater than 0.02 atm).

2. You cannot hold your last breath of cabin pressure air. To try do so would in fact be very dangerous, since your lungs would be trying to support a 0.9 atm pressure difference between the inside and outside. Scuba divers are taught to never hold their breath while ascending for exactly this reason - the pressure difference can rupture the alveoli in your the lung. This can result in gas passing directly into the blood and causing an embolism.

3. But if you stick on an oxygen mask, that 0.1 atm of air (with a ppO₂ of 0.02) has been replaced by 0.1 atm of O₂. So your ppO₂ is then 0.1. Still not ideal, but much better than 0.02 atm.

It's also the starting gun.

When you hold your breath, you choose when to start. That is known.

In a hypoxia incident, you rarely discover the pressurization problem at the very start of the event. It may be well along before you notice it. So you don't know when you actually started "holding your breath".

It would be more of a fair comparison if the failing system went "loss of cabin pressure in 3, 2, 1...” But of course, it doesn't do that!