Same reason there's no fascist party: the two main parties contain a broader range of the political spectrum than in most countries.
From there the question is does the moderate or radical wing of the party gain more influence. The far-right has won the Republican party years ago while progressives still haven't gained that much ground in the Democratic party.
As representatives, this is absolutely the case. However I'm going to give OP the benefit of the doubt and take it that they're taking about the voter base. I myself hold very extreme political views, I feel we should move to a democratic technocracy with a heavy socialist lean and a community service focused punitive system, but as a US citizen my ideals aren't supported let alone championed by my representatives. So I can use my vote 3 ways. I can choose red who actively seek to attack my family and friends. Blue, who will never choose to improve the country, or no one and my vote is meaningless and actively helping whichever side is pressing the most harmful policy.
So alas I am a Democrat. Do they represent me? No. Do they support me? No. Do they want to kill me? No. Out of my very few options, the group that doesn't wish my death is the absolute best I'll see in my lifetime.
I understand what you're saying but it's not clearly written and a lot of people are misinterpreting you.
My understanding is you're saying that political differences in the United States exist within parties (left Democrats vs center-right Democrats), but in other countries those differences would have their own parties. So the political policies of a party reflect which political philosophy is more dominant within the party, rather than a situation where parties with more uniform beliefs are vying for power.