If I have #6"HCl"#, why can't I change the subscripts and write #3"H"_2"Cl"_2#?
The first column is what we mean by having six
The second column is when we draw out what it literally means to have three
#"H"_2"Cl"_2#molecules (and the red means it's a very problematic structure according to MarvinSketch).
The third column is what we mean by three
#"H"_2#and three #"Cl"_2#molecules.
Those are three ways to depict the combination of six
So just know that when we write out chemical formulas, we specify very particular molecular structures. We can't balance reactions using subscripts (unless the reaction had a typo or something).