Because, based on the way they perturb their star, and the details of theirorbits, the upper limit to the planetary masses is less than 0.04 times the mass of the Sun which is the minimum mass of an object capable of self-contained thermonuclear fusion. This establishes the minimum mass for a true star. Anything less than 40 times the mass of Jupiter is therefore defined as a planet. The 'exo planets' are all less than about 15 times the mass of Jupiter which places them well under the minimum mass of a true star or 'brown dwarf'.