Because it actually does mean something, even if we don’t really know what and borders are fuzzy.

When you hear that X is more intelligent than Y, there is some information you learn, even though you didn’t find out exactly what can X do that Y can’t.

Note that we also use words like “mass” and “gravity” and “probability”; even though we know lots about each, it’s not at all clear what they are (or, like in the case of probability, there are conflicting opinions).