Because the electromagnetic waves which WiFi are, is not powerful enough to cause damage to your cells (the proper term would be non-ionizing).
For example, nuclear radiation and scary stuff like Gamma Rays are super powerful and contains a lot of energy, they are also ionizing rays, so they can cause a lot of damage to your cells. When your cells get damaged, they mutate and cause cancer.
Microwaves (Not the electrical appliance, but rather the technology behind the electrical appliance) and radiowaves (like radios) do not cause harm because those waves are too weak and contains too less energy to be harmful.
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