There are not only the reps and dems. There are other political parties.
Some other parties include the libertarians,
the communists, the green party, and the democratic socialists to name just four. There are lots of other groups that are smaller and less well organized.
The reps and dems dominate because they run a system that deliberately produces this result. It is called "the two party system." There are a large number of things that are done quite explicitly to make it easier for the two main parties. This includes things like additional hurdles to get on a ballot, difficulty qualifying for tax breaks, difficulty getting included in debates, etc.
Plus, the reps and dems have party machinery that has been built up over many decades. This includes such things as political relationships among the various levels of govt (city, state, fed), relationships with members of the media, large cash reserves, contacts for mundane things like advertising, printing services, venues for various gatherings from conventions to planning sessions, etc. etc.
So how does it work? The reps and dems have a system where they get their candidates on the ballot automatically, but the libertarians or communists have to get petitions with many names. The fees are paid by the party. The party has existing advertising channels, merch suppliers, TV station buddies, etc. The reps and dems hold their debates without third parties. The party handles the legal aspects, gives their candidate the various tax break documents, and funnels party funding to their campaign. While all of that could happen for third parties, it is constrained.
So a politician with, call them "novel" ideas, finds it vastly easier to pick one of the two existing parties and work within that. So you get "wings" of the reps and dems. The social democratic wing of the dems or the libertarian wing of the reps. And this tends to syphon off people who would otherwise be in a third party. They can get party support from a large party, and get elected. Or they can struggle "in the wilderness" with a third party.
So to get a third party in requires something unusual, even extraordinary. For example, the republicans were a third party that formed over the issue of abolishing slavery.