Why you don't see fireflies during the day?

Because they're nocturnal insects, fireflies [Lampyridae family] tend not to come out during the day. They prefer the pollen and the insects that they find at night. Additionlly, they use their lighted abdomens as part of their mating procedure. The abdomen may be lighted during the day, but not sufficiently visible or attractive to the opposite lightning bug sex.