Most managers do not view "I am looking elsewhere" as a time of negotiation unless it is a highly valued employee. Even then it is wise for the manager to start looking around too because the likelihood of the employee leaving is high. However if the employee asks for a raise rather than saying "I have another offer", then it is a negotiation. Because the manager also knows that if the employee does not receive a raise, then he or she will likely become unhappy and leave.

I agree with your general premise that most things are a negotiable. However it also depends on the situation and how the employee and employer approach the situation. "I got another offer so you should match it." is not a good way to approach a negotiation unless you are a star employee.