Daniel Listhaus explains that
as the Gemara says, it is truly impossible to reward a mitzva in this world
so that
when it comes to those who are being rewarded or punished in this world, it must be that their rewards and punishments are not actually from keeping or transgressing the Torah but rather for doing other things. What are these other things? What is there to be rewarded or punished for if not the based on the Torah? The answer is that rewards in this world are for people who do mitzvos shelo lishma.
And
when we do the mitzvos in this fashion, we are not truly fulfilling the mitzvos in the way they are supposed to be done commanded from the Olam Ha'Elyon, and therefore the reward for doing mitzvos in this way does not come from there either.
