Why can't a sociological perspective on religion "prove or disprove" the validity of particular religious beliefs and doctrines?
A sociological perspective on religion cannot validate or invalidate the beliefs of that religion because the validity of religious beliefs are not within the realm of scientifically provable facts. By using a sociological perspective we might be able to say something about the impacts of various religions’ beliefs on their adherents (though we would have to be very careful to avoid being fooled by confounding variables), but we could not say anything about the validity of the beliefs themselves.
The sociological perspective allows us to think about why things like religion have a place in society and how religion impacts society. When we think about why we have religion, we can empirically show, for example, that people are comforted by religion and that religious belief lowers the level of stress that people feel. When we think about how religion affects us, we can look at the behaviors of religious believers in contrast to the behaviors of people who do not have religious faith. These things are facts that are falsifiable and are therefore able to be subjected to the scientific method.
By contrast, the legitimacy of a religion’s beliefs and doctrines cannot be falsified. Let us think about a religion, for example, that believes that God wants women to submit to men and to be the junior partners in their marriages. We cannot determine in any scientific way whether God actually wants this. We could study the impact of such beliefs on the lives of women, but we could not study God’s intentions in a scientific manner. (We could look at what the Bible says, but scientifically speaking, there is no reason to assume that the Bible contains God’s actual opinions.)
In short, the sociological perspective only allows us to look at things that can be falsified through scientific inquiry. It does not allow us to understand anything about supernatural phenomena such as the existence of God and the laws set out by God.