I see your point but again I'd say it's because of the US's winner-take-all system, as well as 50 states vs 650 seats

Farage posed enough of a perceived risk to the Tories that they moved in his direction to avoid losing votes to UKIP. UKIP never would have won more than a handful of seats, let alone a majority, but by splitting the right vote Labour could have beat the Tories in swing seats

And yes, that could be broadly true of a 'spoiler' candidate in the US presidential election, except that:

1. Only 50 states, and therefore a tiny amount of swing seats compared to the UK

2. more population per state than per British seat. By a whole huge margin. So its not enough to potentially appeal to 8,000 people to 'spoil' a seat

3. The above leads to funding issues. Not only is there more money generally in the US elections, but because you have to flip a big state not a small constituency, you have to spend way way more to make an impact. You can't focus a small budget on one tiny area and win a seat

4. Winner-takes-all means that as long as a campaign thinks it will win a state, and then a presidency, who cares if some counties went to a spoiler candidate?

I'd love to be wrong, and I do think that there's probably also a cultural/historical element to the US's two party dominance. But that said, its just a different system, different processes, different outcomes, different challenges than in the UK