Ok. But please see how many people you see doing that? Many people are able to do that, up the stairs only though, I have never seen anyone going 2 steps downward.
You are right, but running up and down the stairs is a common phrase when telling your kids to stop it. I raised mine in a three-story house and must have used it a million times. They tended to jump from just below the half-way point when coming down. Not run.
Practice and habit. It’s not difficult to take two stairs at a time, but some people have more practice with it than others. Some people do it by habit. People who haven’t practiced it and/or are very used to using every step, may have a bit more difficulty.
It is of course about fitness. Kids, particularly lean kids can manage it as a matter of routine. If you don’t believe it, offer any random skinny kid a couple of bucks and watch.
2 steps at a time works better for my legs – fewer steps taken so they don’t feel so tired when I get to the top. The extra effort to go the 2 steps vs 1 step is not a big deal to my legs.
Without counting, taking two steps at a time tells you something about the total number of steps you climb. Depending upon whether you finish with the leg you started with and whether you can finish taking two steps or have to take one step, you can tell if the total number of steps is divisible by 4 or else has a remainder of 1, 2 or 3 when divided by 4.