Both tend towards authoritarianism on the political compass. It’s about removing individual freedoms, either for everybody (Marxism) or a specific group (Fascism).
This is a false dichotomy, and why you cannot use a simple one-dimensional model for politics. Anarchism and libertarianism are also far left and right, but are both anti-authoritarian.
This is mostly an exercise in arbitrariness. If you arbitrarily choose one dimension or one thing to analyze, you will eventually find something where there are subsets of the left and subsets of the right that agree, but it doesn’t suddenly make those people in those subsets switch to the other side or become more like the other side overall than centrists. Measured across the full spectrum, a centrist will still have more in common with a right or left-winger than they do with each other.