I believe that is because there is a luck involved with this. Like people always knew all along, you could get all 4 in the first roll or you may not get it after a thousand tries, all depend on the luck itself. I got it two times so far and I wagered a bit under a thousand tries and it wasn't bad, yet it wasn't enough to cover all the loss I made neither so I wouldn't be able to say it is the thing that turns it all around, I have actually won enough from outside of the bonus that helped me break even before (now not so much) so I would say getting 4 scatters in a row is not the only way, and you should not gamble just for them alone.

At the end of the day gambling suppose to be fun and if you are having fun that is more than enough, forget about what you get, as long as you are happy that is all that matters and that is all what you should focus on.