Maybe for the same reasons that teachers do. Or maybe for the reasons that parents do (and then deny it.) Or maybe for the reasons that executives do...or sales managers...or movie directors....
But you never told me. On what do you base your opinions?
Gravity and the other laws of the universe were either created by God or ARE God in a sense. They are pretty much immutable and steady as far as we humans know. They don't play favorites; they subject everything to the same set of rules and they give equal attention to every object.
Similarly, I imagine that a God who has a WHOLE UNIVERSE to keep an eye on is really not going to pay that much more or less attention to a quark than a rock or a squirrel or a good man or an evil man or a supernova or a black hole. There are, what, five billion of us, and millions of galaxies brimming with possibility out there. We're all brothers to him, flesh, fire and stone, and we all need minding for different reasons and in different ways.
Sometimes I think it is man's fragile ego begging for a stroking when he imagines that God either pays more attention to the virtuous or favors them or loves them more. Those most desperately in need of attention or uplifting seem to be the ones crying the loudest that God is "on their side". Those who are content with their lot in life seem to be of the mind that God loves all his children equally. That is not evidence of God's intentions, but it does hint at the origins of human belief in God's favoritism.
The only evidence I could point to that God favors all his children equally is that we all end up the same, as dust of the universe. That doesn't take into account the possibility of favored treatment in an "afterlife", but since it's impossible to prove directly that one exists, evidence must come from our physical world.
I'd also say that the fact that good people suffer horribly from time to time is potential evidence that God treats all his children equally. You could probably find a correlation between the "goodness" of people and the quality of the life that God gives them, but it's far from overwhelming.
I'll think about this some more and post again, but I'd be interested to read your thoughts as well.