Any health effects may be cumulative. For example, everybody has cancer cells somewhere in their body. Occasionally one cancer cell, or cluster, accumulates sufficiently - reaches a tipping point so-to-speak, and the cancer spreads, quickly killing the person. That person may have been healthy in every other way for 50 years, but it only took 6 months between the tipping point and death. Do cell phones increase the probability that someone will reach the tipping point? Cigarettes do. Living over a uranium deposit does. We have decades of evidence for those factors, but only 10 years for cell phones. Cell phones are new. And they give off radiation. So why not research it.

On a side note, it IS known that people who use cell phones while driving are at greater risk of being in a fatal car accident.