We are comparing two different things. Yes, historical sciences are different in some ways. We cannot do that in cosmology where we study the universe. Of course, there are theories where our universe is one of many. Not only do young-earth advocates try to draw a line so as to dismiss the evidence for an old earth, but also atheists try to draw a line to dismiss the historical evidence for the truth of the Gospel.
Historical geology is more like astronomy than cosmology.
We can observe many different deposits of different ages and compare them to modern environments to determine how they formed and how they change. We can put organisms in different environments and observe how they respond, including evolutionary changes. Experimental petrology can test how certain kinds of rock form and change.