Because you've never done it before. You don't have experience to learn and remember from. I use to play Grand Theft Auto 4 on my Xbox 360, so I know how to shoot a gun better than I would have. When I got an air soft gun, I would have gotten on the wrong end if it wasn't for that video game. Like I said, it's all about experience.
By repeating (practicing) an activity...our mind registers the process and whenever the same activity needs to be done mind do not need our FOCUS . it will do that automatically. Since repeated activity do not need much of our concentration we start feeling its Easy.
But new activity is New to the mind . ..and it is not yet registered so it demands your Focus and Concentration. But from childhood we are get trained many things so we used live and do activities with lack of concentration (Semi sleep).... all the time..But all of sudden if a new activity come into picture....mind start feeling insecure of the unknown(Because mind's primary motive is to keep you secure and alive) and demands your FOCUS and Concentration. It means you need to WAKE UP and ALERT while doing new activity.....that demand creates uneasiness ...nothing but Difficulty.
Because you have no reference points or "trained" brain neurons developed yet - the first time in nearly every activity you do will be most challenging for this reason. About those brain neurons: example - suppose you're learning to play the piano - if we scanned your brain by a special machine that can "see" nerve cells associated with piano playing in your brain - as you practiced and became more skilled at playing - we can actually "see" changes in the physical structure of your brain. This applies to any activity - intellectual things like learning mathematics to physical things like how to throw a 3 pointer in basketball. The universe is in those three pounds sitting atop your shoulders...