It's not, though. The last time a third party won a presidential election was when Abraham Lincoln was elected; since then, we've had an unchanging two-party system.
The US system of elections is fundamentally broken in two ways: the existence of the electoral college, and FPTP voting. Both need to be fixed before any third party has a chance to win the presidency, and until then, third parties function primarily as poison pills to draw voters away from one of the two dominant parties without any chance of winning.
Oh, and primaries suck, too; they're largely responsible for the phenomenon of Trump, since candidates have to cater to the extremist base to have a chance of winning their party's nomination. But there's no good fix for that.
Anyway, a vote for anything other than your lesser of two evils is simply a wasted proxy vote for the person you hate the most. We have to fix how we vote in the US to have any chance of real change; this is why the person you're responding to isn't doing some wild justification - they're simply recognizing the reality of our fucked-up situation.