A table is not a chair because a table is not a chair. A mountain is not art because a mountain is not art. You're free to use words wrong if you want, but that doesn't make them not wrong.
In particular, if art was so easy to define, we probably wouldn't have philosophical studies such as "Definitions of Art" - https://books.google.ie/books?id=6peJnXr4L5gC
And even the chair&table example isn't that straightforward - https://www.coolthings.com/little-oyster-coffee-table/