Monospace text is objectively less dense, which means you have to move your eyes more. Every eye movement is an opportunity for error. Monospace text only makes sense when seeing exact character counts matters (which it often does in computer code).

One could argue that less density, as well as standardised widths, significantly reduces opportunity for error compared to cluttered text that is constantly varying how it is displayed. Perhaps moving your eyes more increases opportunity for error by 10% but easier-to-parse characters decreases the opportunity for error by 20%?