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Centrism from where? American centrists would be much more conservative than their European counterparts. All this stuff is relative.
Centrism is not always the best position to take on issues. Take slavery as an example: abolition is an extreme position, yet its clear to most that it is the most moral and correct position.
Centrism would be a good idea, if we did not have this many political parties. We have Democrats, Republican, the Green Party, and Libertarians. Centrism, in the simplest of words, would eliminate the point of all these parties and we would have one, whole party that Americans would have to support.
Centrism seeks to compromise with the extreme ends of all ideologies- but constant compromise makes progress difficult, and too much of it will lead to war or conflict between the more radical ends of the spectrum.
Political party affiliations discourage critical thinking by demonizing every aspect of the opposition and glorifying every aspect of fellow members. In almost all situations every side of the political spectrum will have both valuable and poor fundamental beliefs and ideas.
Just because one affiliates oneself with a political party doesn't necessarily mean one can't be a centrist.
The political leaning of a political party is fluid
Political parties are not the only bodies for political movement. Compared to organizations like unions, militias, and other such groups, political parties are inherently moderate because they see electing officials as the best vehicle for their influence. Many groups seek other actions.
Abstainment from a political party does not exclude one from non-centrist political views.