The answer varies from atheist to atheist, but in general, because the theistic claims that a god exists haven't successfully made their case. They haven't provided sufficient evidence.
Often, we find that those who believe in god aren't familiar with what makes for good evidence, let alone qualifies as evidence at all. The theists seem to be convinced that we are surrounded by evidence, and "it is just obvious" that a god is real.
Whenever challenged to provide the evidence, we are typically presented with arguments that are full of logical errors or other major reasoning errors, have faulty information and premises that aren't true, or are vicitms of common cognition biases and errors, such as:
- The placebo effect
- Confirmation bias (preferring information that confirms rather than contradicts one's current beliefs) 
- Priming (power of suggiestion)
- Availability heuristics (if something readily comes to mind, it must be important; or more recent information is important) 
- Imagination inflation (repeatedly imagining a scenario increases confidence that it actually happened) 
- Or a simple misunderstanding or misinterpretation or mis-observation of events.
The profession of "Magician" exists because humans are extremely easy to trick, not because magic is real.
Many atheists are aware of these issues, so we require science, which has major mechanisms for mitigating human error, to better investigate reality. Claims which do not survive scientific scrutiny are not considered credible.
Please note, that as described here, most atheists do not assert that there is no god, but rather we don't accept the claim that there is one - which allows for the possibility.