To describe why it’s difficult to know the true meaning, each of these metaphors on their own are awkward for different reasons. Salt doesn’t really lose its saltiness. A city on a mountain as a light is not very practical other than to be seen from far away. You don’t usually keep a lamp/light burning forever and it will need to be extinguished at some point. These few verses don’t fully function as parables. They are more proverbial. Ultimately, holding onto Matthean themes and Matthew’s narrative may be the best help for our understanding.