There is something in fennel, licorice, and anise that tastes similar, though perhaps not identical. It may seem more that way to those of us who dislike that flavour.In that case "some" have seriously disturbed taste buds. Ditto the people who think fennel tastes of liquorice.
Liquorice and anise are NOT the same flavour at all neither are they related in botanical terms. I like one and not the other.
Just looked it up on Wikipedia. Fennel, anise, and licorice all contain anethole.