Votes aren’t cast against candidates.
While there are two dominant political parties in the United States, every presidential election I’ve participated in has had more than two candidates to choose from. I’d appreciate it if you’d expand upon your point.
Face it: there are only two candidates who realistically have a chance at winning the general election. It’s been that way for every US election we’ve seen.
If you vote for someone who doesn’t have a realistic chance of winning, that’s about the same as just not voting at all.
So you really have 3 choices: candidate A, candidate B, or indifference.
And there are two possible outcomes: candidate A or candidate B.
If one of those outcomes is at all preferable to the other, (e.g. either A is “better” or B is “worse”), it’s strategically best to vote for the main candidate you prefer, since that increases the chance of getting your preference of the two outcomes.
How many presidential elections have you participated in where more than two parties received any electoral votes at all?