Although individual atheists probably differ, as a whole, I think it is because they don’t perceive sufficient evidence to conclude there is a G-d. Thus, even if they cannot completely rule out the possibility of the existence of G-d, they don’t see any reason to believe in an entity that may or may not exist.
For others, they perceive the problem of evil as insurmountable and therefore, while G-d may exist, He cannot be a totally benevolent G-d if He allows pain and suffering to persist. Their reasoning might be that no amount of eternal reward in heaven can erase the pain and suffering experienced by so many in the present. Therefore, they refuse to believe in a G-d Who is either not benevolent or not powerful enough to remove the suffering of the moment.
For still others, perhaps they would like to believe in G-d, but which G-d? Since there are so many different religious versions of G-d, they might feel it is a futile endeavor to choose only one.
Finally, as others have mentioned, some atheists may be going through a phase in their spiritual development and are perhaps influenced by friends and secular society in general. They may be convinced that belief in G-d is both a sign of weakness, a crutch, as well as fraught with hypocrisy since religious people are not necessarily good people.