Why do scientists believe or disbelieve?

The same reason as everyone else.

"In Science vs. Religion: What Scientists Really Think, Rice University sociologist Elaine Howard Ecklund comes at this question by means of a statistical survey. Between 2005 and 2008, Ecklund and her associates randomly selected researchers from across seven natural and social science disciplines at twenty-one elite U.S. research universities. . . . Ecklund concludes from her research that most scientists do not become irreligious as a consequence of their becoming scientists. 'Rather, their reasons for unbelief mirror the circumstances in which other Americans find themselves: they were not raised in a religious home; they have had bad experiences with religion; they disapprove of God or see God as too changeable.' The disproportionately high percentage of nonbelievers among scientists (as compared to the general population) would appear to be the result of self-selection: the irreligious seem more likely to become scientists in the first place."