Wouldn't a user take the same care on the iPhone 8, as he would an iPhone X?This is true, but....
Durability can be tested objectively, but the weight that it is given in an overall subjective.
If a user plans on using a case, or has a habit of babying their phones, then that user would feel that the weight given to the total score would be too high. If someone needs a very durable phone, then the weight given would be too low for them.
I am not going to bash CR for how the prioritize ratings, but just say that they have to pick something.
I had a good case on my iPhone 6, and I have a good case on my iPhone X. I think very few people buy a new phone (8, 8 Plus, or X) with the intention of abusing them. Given an unfortunate accident, the 8/8Plus is more likely to survive, than the iPhone X. Glass just doesn't absorb shocks, scratches and abuse like Aluminum can.