Probably the same reason why the life of a cow is apparently less significant than that of a human.
There is a hierarchy of 'life'.
Those who try to say 'All or Nothing' when it comes to harming a sentient being are either ignorant or simply trying to justify their meat-eating habits.
It probably requires a different breed of humanity to actively choose to get involved in any vice-related industry.
Or maybe I will do it too given enough incentive.. who knows.