A first thought is it is hardwired in to the brain. We evolved in this world, and if it just so happens that in this world, 1+1=2 and associated concepts have a great deal more utility than 1+1=3 and so on, then it shouldn't be too surprising that the brain that has evolved, the brain that has survived, was the one wired for this thought, and not one of the alternatives.
For the life of me, I'm missing a lot of the subtlety about this question as I write this post. But a second thought, woefully undermotivated by this post, is: it is wired into the WORLD. 1+1=2 is pretty much the way of the world. Having been wired into the world, and with consciousness being so intimately linked to the world, conscious minds just can't really operate unless they perceive 1+1=2. To the extent they are able to get in the neighborhood of 1+1=3, those minds are on drugs or otherwise having their tight connection to the world attenuated.