So that various sets of ideas can be tried as experiments. Also, so that groups of people with different values and ideas can live in their own worlds and not have to reach consensus on everything. I believe a lot of the division in the country could be solved by returning more decisions to state and local levels. Only centralize what the constitution requires.
Mechanically, America could convert to a unitary state with a Constitutional amendment. Practically, very few want that. American states individually command resources on the scale of rich nations.
That's literally impossible given the way our Constitution and government is written. The only way would be a civil war by those who don't like the ideas of "states" and I don't think there are enough of those individuals.
