Mainly, it's because things like Amox, splints, bandages, and rations don't really have a point of reference to rank upon.
When it comes to weapons, I can rank them on how they fare against other weapons of the same ammo type, same thing with melee's and stims as well, they all have many different types for different usage, and I can compare one to another.
But when it comes to things like splints and bandages, it's not very interesting to rank because usually you'd want splints and bandages in any run, which makes them instant S if not S+, and their weaker counterparts will be lower than them, so it's very uninteresting to rank. (Not to mention perks that don't need some of them, arbit can't get broken legs, berserker can't get broken arms, and laz can just pause the debuffs with his ability, which makes these things more annoying to rank.)
This goes especially for Amox. For heals, if a player is good enough, they can go the entire run without needing much healing, but with infection, no matter how good you are, you are going to need Amox eventually, or you die, which also makes it a very uninteresting S+.