Because pure objectivity in social sciences is a myth, a utopic state at the best of times? We all have biases when it comes to deciphering human behaviour -- the best scientists will try to shed them harder than others, but they'll likely still retain some.

On top of that, you have to consider that social sciences (including economics) are "Schrodinger objects": as you observe and describe them, they change. Once you understand that, you can leverage the dynamic for personal or political gain, which is extremely common.