Because it’s laughably wrong and misguided. By that definition no center can exist. Which is obviously and observably patently wrong in every sense.

Also left is not defined as communism. Left is defined as socialism. Communism especially big C communism or ML is an authoritarian socialist ideology. There are also non authoritarian socialist ideologies. Including other forms of non-state communism. I understand your misconceptions. Believe me as an American and someone raised in the west myself. We are all purposely miseducated on them. But no matter how comfortable it is. It is still incorrect and misinformation.

For example let’s refer to a few of the ideologies that I mentioned before liberalism, Democratic socialist, and social Democrats. Democratic socialist exist and are pretty centrist. By your definition they do not exist. Yet they do exist. Social Democrats are to the left of Democratic socialists. But they are generally against ML communists. Which by your definition makes them not left. But social Democrats are also generally against capitalism. This does not make them fit under your left or right spectrum. This simply marks a gap in your understanding. And nothing more. Because they are solidly socialist. I hope this can help clear up the confusion for you.