Because looking at a chart to make a decision is subjective. When running a fund, you want subjectivity to be cut to a minimum or better yet, completely eliminated.
Subjectivity is not a question of tool but a question of mind.
If you work with graphs (with or without indicators), price, volume, order book, facts : they are what they are. Real. But your own analysis is subjectivity.
If you look at graphs or numbers, and just go with the flow : you are closer to objectivity, and at the end profitability. No emotion. You trade what you see. You win. You lose. This is routine. But at the end of the day, week, month, year ... you get more winners than losers.
Alas, we are not robots but humans, we usually want to "interpret" numbers or graphs in order to fit our desire : we "feel", "hope", "anticipate", "regret", "shall", "can't accept", etc.
Emotions are the roots of subjectivity, not graphs or any other tool by the way.
CM