Because that directly conflicts with making the highest profit possible (still legally).
What’s sad in these discussions is that it’s both true that Amazon is a terrible employer and that it often pays more than alternatives for many workers. This seems like a failure of policy (ex. Enforcing living wage standards) and American business well-being at least as much as a failure of Amazon to “do the right thing.”
Edit: Cracks me up that I got downvoted for telling the truth about capitalism.