I have a question about a claim in Richard.A.Jhonson's multivariate statistical analysis Chapter 1, page 13. So following the example in the picture, we have two variables $X_1$, $X_2$ which represents total salary of the baseball team and the ratio of win matches over the total number of played games respectively. According to the data, it seems high payroll have a higher success ratio. But it says that
" this cause-effect relationship cannot be substantiated, because the experiment did not include a random assignment of payrolls. "
I don't understand what it means. Does this mean that the data about payroll is not a randomly selected data or that each player's salary in a team is not random? Why can we not say that the wage and the success ratio is somewhat correlated?