Because "liberal arts" is considered one word: a plural noun. It only means what it does as a plural noun. You can write it like you did, but it's rarely done, and you did it the worng way when you said, "I know what liberal art is." That would be art that's liberal.
It can be written like this though: "Should walking be a liberal art?" But few have used it and it should probably be avoided.
You should read about the history of the term "liberal arts." I suggest Wikepedia.