They're not 'defining' art.
If you want a definition for art, here's one: art is cheese and cheese is art. There.
That's obviously a false definition, for we recognise that some things are art yet not cheese, and some things are cheese and not art.
We don't have a rival definition in our heads, do we? After all, how would that rival definition show there to be anything wrong with my one?
We already have the concept of art. When we try and 'define' art we are not 'creating' the concept, rather we are trying to capture it.
Big difference.
That's why my definition above should be rejected. Art and cheese are not synonymous. My definition made them so - but so much the worse for my definition.
So, let's be clear about what way around things are. We have the concept of art - a concept given to us by our reason.
We then wonder what it is that qualifies something as art. We have already started recognising some things as art and some things not - so we don't need a definition in order to do this - but we are just wondering if there is something systematic to it.
Hence we speculate about what may qualify one thing as art and another thing not.
But again: we don't need the definition in order to be able to recognise that there is art in the world.
You don't need a definition of a mountain in order to be able to recognise mountains, or a definition of 'you' in order to recognise that you exist. We can try and formulate definitions, but it is a mistake of the first order to think that our definitions are what's in charge.
The archaeologists in my example are less likely to make this mistake.
Other examples of this mistake: thinking that whales are mammals; thinking that peanuts are not nuts; thinking that tomatoes are fruit.