Why is it hard?

Due to the game including a cue stick, a ball to hold momentum and collide with the other 15 balls, the game can have lots error.
That's not even including that you have to aim those balls for 6 targets.

The error can begin with the very first move, the cue strike. Due to a sphere having near infinite sides, it causes human error margins to be, well near infinite.
Following that, if the white ball is not hit properly (which will be the case almost all the time) it won't hit any of the other 15 spheres properly.
In the end, with near infinite sides on 16 balls, excluding human error, there are 16 times near infinity reason for the game to go wrong.

Lets put a situation in place.
You play your first game of billiards, and every turn you make a ball in, winning the game.

Lets do the math of that match.
You hit one ball in at a time, decreasing the balls that can be interacted with, adding 15! for the entire game.
There is also only 6 holes, lets assume they cover 5% of the rims around the table, that's a 1/20 chance for a ball to make it in (only if aimed perfectly).
Then there is human error, lets assume a sphere has 1000 sides for this, and 10 of those sides will send the ball in the direction you want.
That means you have the chance to hit the ball perfectly 1 out of 100 times.

This all means you have a 1 in 3.8e16 chance to have this situation happen (assuming this is your first time playing and your aim was imperfect every time).