God must have forgotten to install mine and the millions of other atheists who aren't just missing this "instinct", but have no desire to develop it.
No, it is there. The search for happiness that all humanity strives for is the instinct that I am addressing. That happiness derived from this world is fleeting, temporal, at best directs the mind to seek a source for eternal perfect happiness. That happiness is found only in the reunion with the divine.
That you and other atheists reject the existence of such a happiness as a fantasy is your choice, but, even as it does so, you exist in this world and your mind ever searches for a better happiness (a more lasting one) as it anticipates the loss of any happiness you might momentarily hold. This is human nature and the nature of worldly happiness.
You may say the faithful believe and search for something that doesn't exist.
The mind of the atheist, however, is no different from the religious minded in this sense: it wants what it wants--and that is permanent or lasting, perfect happiness. It is another part of the atheist's mind that "corrects" the desire and tells one such happiness does not exists and to settle for ephemeral happiness.
The instinct or desire for perfect happiness is there. What you choose to do with it is dependent upon a will which is guided by your intellect. Your intellect does not admit the existence of God, thus your desires for perfect happiness are rejected as unreasonable.
"My heart was restless until it rested in you [God]."
St. Augustine