Because there are apple seeds in the core.
Eventually you plant the seed and even later on a sproutling appears.
You take good care of the sproutling and it becomes a tree.
When the tree continues growing, apples start appearing.
Then you realize, the apple apparently had ten apples and more from the beginning!
So from then on, you make a never ending chain reaction called "the apples seed affect"
I hope you all liked my answer!
The end!
) 