> It's because formulas have a convention, where the LHS is a single term naming the value you want, and the RHS contains the terms for values you have.
I've seen that, too, but there is usually a reason for writing it in that form. The most common would be that you have a different equation with 2K in it, and so you want to make variable substitution simpler. Alternatively, if you are reading older papers, typesetting inline equations that don't fit in a single line was painful. For that reason, a formula might be rearranged to avoid needing any fractions.
While I don’t think the terminology is explicitly standardized, I think most people in the relevant fields would call that statement of the ideal gas law an equation but not a formula, the latter being a special case of the former.
Can you find any reference to the idea gas law as a “formula?” As far as I can tell, equations without a single variable on the left side are referred to as simply equations, while solutions of such equations in terms of one variable are referred to as formulas. This seems to be the case for every well-known identity I can think of, like the quadratic equation/formula. Can you think of any counterexamples?