[AmarettoAndCode]

In my mind, it's the difference between having a singular thread of execution vs multiple threads.

The people in the aisle are units of work that need executing. In back to front, only the unit that is right at the front can actually be completed since only she is blocking the people behind her, who are in turn blocking the people behind them.

In a random setup, say the first person goes right to the back of the plane, there's every likelihood that the person behind her is not sitting next to her, which means that 2nd person can immediately reach his seat while she does the same..

Multiply this a good few times, and you'll see that a number of people proportional to the total number of passengers will always be getting seated at one time instead of just 1 or 2..