Einstein declined, saying he lacked "the natural aptitude and the experience to deal properly with people" to do the job.
Einstein on Decision to Not Accept Presidency Offer:
"I am deeply moved by the offer from our State of Israel [to serve as President], and at once saddened and ashamed that I cannot accept it. All my life I have dealt with objective matters, hence I lack both the natural aptitude and the experience to deal properly with people and to exercise official functions. For these reasons alone I should be unsuited to fulfill the duties of that high office, even if advancing age was not making increasing inroads on my strength. I am the more distressed over these circumstances because my relationship to the Jewish people has become my strongest human bond, ever since I became fully aware of our precarious situation among the nations of the world." (The Albert Einstein Archives, The Hebrew University of Jerusalem)