Jhieminga is right. The specific question is "why don’t we use a factor for wet runway on take off." Unless there is something more to the question that we don't know, the answer is that if the surface is unpaved, grass (esp. long wet grass) or flooded, we
should use a factor (easily researched and in the POH). If it's paved we don't need to worry too much unless there is standing water. Even flying off packed snow made little discernible difference to the take-off distance [unless there was a crosswind
] but landing would be a whole different thing.