Can somebody please explain to me why and how bisexuality exists

As in what causes it? Why has it not disappeared over time? What preserves it?

It doesn't if a woman calls herself "bisexual" she's either playing around or is trying to hide the fact that she's gay, same for men however men who are sex addicts will go for both sexes and get the same satisfaction

Social animals survive better if they can maintain group harmony and cohesion. Doing fun things together helps maintain group harmony and cohesion. Sex is fun.

Humans are, basically, the only organisms (that we are aware of) that actually *know* that sex=babies. Other animals, essentially, have sex because it feels good.