Why is the definition of opportunity cost "the benefit of the best alternative choice" but not "the benefits of all alternative choices combined"? What is the rationale behind it?
This is because it does not make sense to compare chosen options to all other alternatives that are mutually exclusive. For example, if you have 5 choices A, B, C, D, E, you can choose one, and C is the most preferred option it does not make any sense to compare picking C to jointly picking A, B, D and E. Only one of the other options can be pick so you should use best of them for comparison