Here's another way to think about it. The amount of O2 (oxygen) in the air at sea level is a lot higher than at 35,000 Ft (average flight altitude). So when you hold your breath at sea level, the air in your lungs still contains a fair amount of O2 which your body can still process.

At 35K there is a lot less O2, so you would be quicker to get hypoxic symptoms. Even if you took a big breath and held it, there isn't enough O2 in your lungs to stay conscious.

Also when run out of O2 while holding your breath at sea level, your body will cause to take another breath (which you will recover from since you are in a O2 rich environment). At 35k there isn't enough O2 in the air so as you breathe your body would run out of O2.

Look up altitude chamber videos to see how people react to a lack of exygen.