I would modify this slightly. We do experiment with humans; we just cannot do _certain kinds_ of experiments with humans because of ethical considerations. We constantly do clinic trials, which are essentially experiments with a high degree of controls and randomization.

I think probably the major challenge for the social sciences is the nature of the phenomena under study. Psychologists, sociologists, and economists aren't studying physical quantities that can easily be measured. Instead, they typically study abstract and intangible variables. Measuring these accurately is very difficult.