Lots of people like the abstract idea of another option. There are, on fact, other concrete options, and people don't tend to like them as much as the abstract idea of an alternative, even before factoring in tactical concerns.
Now, to be fair, part of that is a product of the electoral system--people who are likely to be competent for the office are unlikely to run as a third-party candidate, given that they are likely to understand how the system works.
Now, to be fair, part of that is a product of the electoral system--people who are likely to be competent for the office are unlikely to run as a third-party candidate, given that they are likely to understand how the system works.