Why are Bikes always Stolen

I see many bikes on the street, sometimes locked, sometimes not, but often they disappear very fast. I tried to lock my bike with a strong lock, but I came back and it's gone. This makes me very frustrated because bikes are important for going to work, to school, or just moving around.

I think maybe city can make safe place for park bikes, or put cameras near the street. Maybe more light in street help too.