If you only see the center overlap you are either not a USA national (looking from the outside in) or a disgruntled US voter.
There are actual differences in policy platforms. However the ''management policies'' of the party's mechanisms are of a similar vein.
The United States is not governed by a Parliament and by the parties' tickets.
The Framers (of the constitution) set up the Electoral College to ensure that radical political demagogues ( like an Adolf Hitler) would not rise to power with a minority vote split between 4 parties like has happened in Europe (and globally) in the past.
The Republican Party was a third political party that with Lincoln's assent to the presidency
pushed the Whig party aside and then over the cliff. Proof that a new party could become a reality.