Let's imagine I have some coins that I stole during Evolution's demise. Are you willing to swap all of your coins for mine?

Or put differently: if I borrow your car for a month, and at the end of that month I give you back the same make, same model, same year, same colour car, is that ok?

Bitcoin isn't fungible for the same reason that you won't appreciate me giving you some random car in exchange for yours, regardless of the properties of that car.